Mark made a rowing seat for our BRE. He used 3 layers of ply for it that he laminated together. The top two layers were long enough to overlap the seats slightly. The bottom layer was slightly shorter so it went down into the gap between the seats. That stopped it moving backwards and forward because the seats taper on the BRE. Mark made lots of holes in the middle layer using a drill and jigsaw to make the seat lighter. He stuck the layers together with epoxy and put a layer of epoxy on the complete thing, then varnished over the top.
It worked really well - I rowed our BRE for several miles on the raids - but it sounds like you might need to design the BR20 one so that it sits a bit lower down in the boat. Mark says you might not need three layers of ply - he only used that many so that we could sit high enough on the BRE.
Mark also says you need to make sure you have long enough oars, as that’s what makes rowing more efficient!
