I've been looking at the pilots seat for the SE5a and references are a bit lacking. The seats that I see in references are from restorations or new build SE5s. They look a little too "Comfortable" for WWI standards. And they also look very non-supportive.
Here are just a few examples:


As I look at the basic build of the seat I see one horizontal slab with a backboard. I'm guessing it was up to the pilot to bring his own cushion. Were they wearing chutes by then? I can see where a "chair could be afixed to that seatboard.
Without better references my plan is to do the horizontal seat board with backboard and place a cushion on the seat and one "nailed' to the backboard.
Regards
Steve