The balanced modulator works! I took the design from a 1960s transmitter because the carrier crystals and sideband filter I got off eBay came from one of them. But I could not get all the parts so I had to alter the design a bit. Today, my little toroid cores and enameled wire came in and I wound a toroidal transformer and added it to the board.

I got 42dB carrier suppression, spec was anything over 35dB is good enough. This was the most intimidating part of the build. I have more challenges, but this scared me most of all as I simply could not get the original RF transformer nor could I find any specs on it. The math seemed to imply that two windings of 44 turns of #26 enameled wire on a T50-2 core would work, and it did. (The schematic is of an SB-400 and the yellow box is the original balanced modulator.)