This looks great man, and is a clever use of everyday available materials.
-functionally, the galvanized sheet will work. Im too anal/OCD and I personally would call my local metal shop and get a piece of 0.040 or 0.062 thickness aluminum, it will be flatter, prettier and cleaner to cut and work with.
-yes, just glue the monos down with thermal compound/epoxy.
-I used to sand to 1000 grit wet/dry using a $10 harbor freight palm sander and a spray bottle of water. now I just go to 400 wet. and its all good.
- GG posted a nice tip on another thread. if you are tapping for 4-40 screws for your cob holders, dont use the bit it comes with (5/640, use a slightly larger bit, like (3/32) it will have plenty of thread "meat" and be more forgiving
- be sure to tap extra holes for COBS u plan to add later, this way you don't have to disassemble the whole think to add more later.
-DONT SAND THE BACK OF THE COB.