I had saved some zee brackets  from my RV 7 I had drilled wrong. (The 10 are already made and you don't have to drill them) Since they already had some nutplates attached I think I can use them to build a couple of mounting plates for the ELT and Strobe Power Supply.

I think the plates can be installed along the bottom J-Channels next to the battery mounting box and be riveted to the belly skin.