The seat showed up, immediately there was a problem, it doesn’t really fit. Its a 16" seat but thats a lot closer to 17" once you measure at the widest points.
Hacked the tunnel out this morning then got to work remaking it to fit people.
Realized that more clearance was going to be necessary so the bars mount along the rear bulkhead got moved 1" closer on each side. The bracing is 3/4" rather then the 1" for weight and marginally more interior space.
Finally got around to triangulation of the outer chassis bits, its amazing how much stiffer it already is with the bars tacked in place.
And now the seat ”fits”, albeit 1.5" off-center from the compartment.