Finally bit the bullet and decided that I'm going to install the com antennas in tandem with the fwd 1 just right of the tunnel, 6" aft of the spar carry through.
Once the decision was made, I bent up a .032" doubler out of some scrap I had, and made a template for the hole spacing of the delta pop antennas. Tied the doubler into a floor rib with some LP4 rivets, and attached it to the belly skin with 4 MK-319-BS blind rivets. I could have gone crazy with a bunch of driven rivets, but since I was by myself and wanted to get this done before I left town again for a few days, I elected to just go with 4 holes that I dimpled with my pop rivet dies and pulled the blind rivets just to tie everything together. The antenna mounts via a couple of studs and nuts, so that will give significant clamping force right at the antenna base and I don't think it really needs any more rivets than this.
This all took a lot longer than it really should have, but in the end, it turned out nice.