So, after a bit more than a year, I replaced the cracked backing plate in the other ERRC underseat pack on my Tour Easy. The first plate held up much better than I expected: hasn’t cracked or poked through the pack fabric.
This repair followed the same outline, including cutting off the ripped netting on the outside of the pack and marching the pack into the clothes washer for a spin with a few shop rags. Reassembled everything, put it back on the bike, and … the new aluminum extrusion across top of the plate smacked firmly into the water bottle holder clamped to the rear of the seat frame for the amateur radio.
The extrusion is the lump running horizontally, just under the seat cushion. The corner of the pack extended rearward (left) of the water bottle holder’s black plastic body.
The original flexy plastic pack plate simply bent out of the way, but that’s not going to work now.
So I loosened the clamp, moved it a bit more to the right, and tightened it up again. I’d originally located it at the far right end of the straight part of the seat frame, so it’s now edging into the curved part that eventually forms the right side of the frame, but it’s good enough.
My shop assistant says she wants another water bottle holder for an actual water bottle on her bike. I say she should just go to the shop and make whatever she wants, then install it. Negotiations continue…