There were two pieces of wood that connected near where the rear spar pass through is to the bottom of the aircraft that the back part of the seat would also rest against.  I cut this out previously to make room for a seat that lets me go back a few more inches, so I'd fit in the darn aircraft.

Were those diagonal pieces strictly for the seat or were they part of the structure of the airplane?  If so, I can probably make new braces to straddle around my new seat frame to connect to the old place they did.  

Also just to be clear, I'm talking about the inboard brace, not the outboard one on the side wall of the airplane.  See the picture.  I cut the red circled piece out, but the ones on the plywood are still there.

This is an existing airplane you're working on, correct? I have a pretty good memory of the plans from when I built my plane and I don't remember the two slanted members inboard of the fuselage sides. My guess, if it is an existing plane, that the builder thought they would be a good thing to add to support the seat back. My vote is that they're not necessary, but just my laymen's opinion.

Lynn

They're not in the plans.  If a stiffened seat back is desired, glue them to the seat back.  I think the seat is supposed to collapse in a crash saving your back.  A broken stiffener could go through a beloved body part too.

Ok, good. That makes me feel better.  What I wound up making was this, an aluminum tig welded frame with some thin plywood screwed on.  A friend will be sewing some very comfortable, soft seat cushions to attach to it.
It is on hinges for quick access under it, and also has two release pins to just pull it out entirely.