we used to replace the flaps on our Flatacraft force 4s with a piece of Hypalon laminated to a section of of old inner tube, with the inner tube on the hole side.
It worked much better, but only until the first tiny piece of debris got in there and held it open.
I converted the last Force 4 I had to an elephants trunk.
RIBBED For extra pleasure.
Member of the ebay Blue RIB cover club.
Member of the Bombard 380 Aerotec club