I have a sensor that I want to use to make the exposure meter. What should I use to make the dome to use it as incident meter? I prefer something small like 10 or 15mm.
For PIR sensors you can buy separate domes. These will probably work perfectly in this application and seem to fit your requirements. The small type has a diameter of about 5mm.
Yes I think a used dome is a good idea. I have a Minolta dome that has the bayonet broken. I tried it out and it works great. Now I have to figure out how to put all stuff together to build the meter.