If you can't find a momentary one, a regular on/off will work just to get you your codes . You will just have to switch it on/off for the required 2-3 secs.

A clear plastic film can makes a good chassis/container for the tool. You can run the leads out a hole in the bottom and mount the sw. in the cover. Can still see the led through the clear plastic and it keeps everything contained nicely.....
Have also used a plastic test tube and even a small plastic flashlight case with the led behind the lens.....
Spend a little time on making it a permanent tool , as you will use it many times....