I picked up a folding camp seat like this
https://www.winfieldsoutdoors.co.uk/...kaAvxGEALw_wcB from Go Outdoors, think it was reduced to £10 at the time. Its actually really effective.
It straps onto the bench and you adjust the side straps to make the back more or less rigid.
Not the same as a fixed seat but provides enough back support to make multiple hours in the F-rib much more comfortable and folds flat so easy to store.
I actually use it when pic-nic'ing and stuff as well now.
Cheers,
MM