It's not too hard to add a small DC fan to a computer mouse. The plastic housing for a mouse, especially on older ones, is usually wide open in the back. Put a fan in the back to keep your hand cool while holding it.
I got this idea from a bunch of places online.
This is originally by metku.net http://metku.net/index.html?sect=view&n=1&path=mods/rottaflekti/index_eng
What you'll need
-A computer with a mouse .... (who knew)
-Small DC fan... (size is up to you, I don't know how big your mouse is)
-about 5 inch sqaure wire mesh
-modeling paint if desired
-small electrical switch
-pliers (might not but good to have)
-LEDs if desired
The first thing you'll have to do is find a computer mouse that you don't mind taking apart/breaking/never getting you work ever again. I suggest using an old one to make sure you can do it first, then step up to one that you might use all the time. Here are some rudimentary steps to follow.
1.Open up the mouse cover. See what it looks like inside and see how much space you have to work with. Now would be a good time to buy a fan if you don't already have one, one that fits. You should see a few wires attached to a small circuit board some where... this is where we're going to draw power from.
2.Once you find the power supply jack I'd recommend slipping the leads to your fan in the different wire ports to test the power. I'd do this while the mouse isn't connected and then connect up the mouse. You might use a multi meter to get the correct voltage for your fan or at least know what the voltage is. Verify that the fan works.
3.You now want to prep the mouse case to accept the fan. Use the dremel tool to cut a hole in the back plate. Make it big enough for the fan to blow air through, but not big enough for the fan to fit through. Make sure you can see the blades of the fan completely, otherwise your hole is too small...(I made mine a bit too small if you can tell in the picture.) It will still work none the less. The next hole you need to make is for the switch. Make the hole along the edge of the case, where the two halves meet... probably somewhere where it won't bother your hand position. You've got all the holes made.
4.You now need to make a grill for your fan. I cut a piece of wire mesh (very fine wire mesh but any size will probably due, use your best judgment) about half an inch on all sides bigger than your fan hole. This gets kinda tricky but the best way to do it is to press the mesh into position until it comes through the hole and contours to fit the mouse case. It should be smooth with the outside of the mouse case. Epoxy the mesh wire in place when you are happy with the fit.
5.Mounting the fan - The fan may fit perfectly into the position, it may not. Try to fit the fan into position without epoxy and close the case... if it fits you are ready to mount it, if not, try trimming the fans outer wall down until it fits. Now epoxy the fan into position. Make sure the blades do not hit the mesh or the case, also make sure to maneuver the wires out of the way close by the power supply.
6.Hard wire the fan and switch - This can be a bit difficult, ideally you want to solder the wires into position. Use the multi meter to find the right voltage for your fan, it should be fine, unless you are worried about over powering the fan, which i wasn't. A small tipped soldering iron and a soldering stand with holding clips is ideal. I just used some tape to hold the leads in place loosely making sure not to melt the tape. Try to use the shortest amount of wire you can to minimize the extra wire you have to stuff in the case which could get in the way of the fan. Include the small switch in series so you can turn it on and off. Any LEDs should be installed now.
7.Close the case - this is the trickiest part. Once you get everything inside, the switch lined up, the wires out of the way, you need to epoxy it together. The most important thing is to keep the wires away from the fan. Once it's closed you won't be able to move them around too easily. Be careful not to epoxy the switch open or closed, not a good thing. Let the epoxy set completely, and then test it out and see how it works!
8.Clean up and paint - The epoxy might look kinda bad to you, just take some sand paper to it, and then add some paint, pretty easy. I used some modeling paint, there might be better paint out there but I'm happy with it.
Sources and inspiration
There are a bunch of sites I looked at online, but it mainly comes down to your own design. It depends on your mouse and how much space you have in there.
I read somewhere about a guy making a mouse heater. Basically he put a bunch of resistors in series inside the mouse and epoxied them to the case where you hand is. It warms up the mouse pretty good I bet, but it's a bit of a pain to do the circuits. He had to modify his USB card to be able to handle the increased load.
I also read about someone eliminating the clicking sound from the roller on top, you might think about doing it since you're going to be opening the case. I'll try and dig that up.
Removing the clicking noise from the scroll wheel should be fairly simple, first remove the casing on the mouse, the wheel is usually held onto the hub by pushing against the case, on most generic wheel mice to inside of the scroll wheel looks something like a socket wrench, there is a small spring arm that fits inside the wheel that causes the clicking as the teeth on the wheel go past it, this spring should be easy to remove, it's usually just held in place by it's own resistance (pushing against a plastic tab or such, and the wheel) or inserted through the plastic by the wheel, slide the wheel off of it's hub and the spring will either fly off on it's own or you may need to brute force it with a pair of needle nose pliers, put the wheel back on and put the mouse back together. I've never tried this on optical mice so I'm not sure about how much room you have in those.
Instead of using the screen material maybe you could dremel a grill or a series of holes into the plastic case?