I'm building one as well, as small and light as a piezo pickup is, couldn't imagine it having any negligible effect on sound quality, although mounting it on the underside of the cone will probably give you the closest to an amplified resonator sound versus mounting it somewhere on the soundboard which will pickup more string vibration transferred to the soundboard like regular CBG's giving less of a res sound, my theory anyway. I'm going to try and mount one directly under the center where the saddle/puck will be mounted.
Love to see pics of your build as you go.
I normally use 27mm discs, but if you are going to mount it to the metal, I find them a bit hot... 20mm seems to work better for me.
I do a lot of Paint can Reso's... I mount the piezo under the edge of the bridge. I get lots of twang..
The best place I've found for mounting is inside the cone in the center directly under the biscuit. Let there be a little slack in the wiring harness.
Thanks everyone for the great info. I mounted it centered on the inside of the cone. Embedded it in a little clear silicone sealer. It sounds fine to me. Sounds lot like my old squareneck dobro.
Post a video! we'd love to hear it!