Why bother making one when you can go to O'Riley's auto parts store and get an excellent bore light. They have a very small diameter flashlight that is just a bit over 1/4 inch (maybe 3/8") in diamter. It i s a very bright one. I use it all the time to examine my bores. I have attached a string to it and I lower it in with the light facing me so I can clearly see the bore as it slides down. With the string attached, I can control where it is in the bore. Then, I remove it and turn it around and examine the breach. It makes an excellent bore light and is not expensive (less than $10) and it fits almost all sizes of bores. It will fit my .40 and larger.