1. Checked all clearance issues with transponder by installing baggage floor and baggage wall. The clearances are tight (1/8”) but it will work fine.
2. Tightened backplate to rack.
3. Finalized positioning of wiring harness where it exits the rack connector. Added anti-chafe plastic tubing around transponder harness so rudder cable has no chance to contact it.
4. Formed all transponder cables into a nice harness. Added Adel clamps along under-floor support structure.
5. Finalized bundling of ELT harness wires.
6. Fabricated transponder coax cable. Connected one BNC connector, then climbed in the fuselage, routed the cable, and connected the other BNC. Routed cable to stay away from rudder cables and other hazards. Secured cable with various kinds of clamps.

Other than sliding the ELT and transponder into their racks at the appropriate times, and connecting the elevator pushrod, I believe there is nothing left to do behind the aft baggage bulkhead. Hooray!