[SteveB]

Chuck
Here is a closeup of the bracket, it extends to a lower fixing out of sight behind the driveshaft. Hope this helps.

Ron
I could probably get a 'Solidworks' drawing if they can use it.
I have put an extra stiffener where the back and base joint meets. I expect the seat will always be hard against the bulkhead , however the extra strength would be needed if mounting on runners