I've seen hard sided dingys on the fordeck from time to time with what looks like poly-foam cut out to cushion it. What has anybody used for this purpose and where did you get it?
Search Amazon for "kayak blocks" and you will see an array of closed cell foam blocks that are meant for mounting a kayak to the roof of a car without roof racks but that can also be used to cushion a smaller dinghy on the foredeck. Some even have slots cut in them to run straps or lines through them, which makes it easier to hold them (and your dinghy) in place.
Closed cell foam cuts reasonably well with a saw (hand saw, table saw, jig saw, or circular saw-- just do not use a router as that will tear it up to bits), so you could cut channels into the blocks to fit along the edges of your overturned dinghy where they meet the deck or cabin top.