On my motor mounts, I used slightly larger thin ply circles I found in the craft section of wally world, rolled some thick paper stock (like poster board) I had laying around over it and glued it together, then glued the ply in the leading end after cutting a hole for the wires and cooling air to enter on the bottom half.
I then cut thin pieces of foam (the foam my DX-A came in I think) and glued 3 strips spaced around the motor and squeezed in into the tubes.
Static running them with the props didn't result in any movement, and if I need to, I can just pull them back out. I wouldn't try this as a tractor setup, but as a pusher, where are they going to go anyway? I wasn't REALLY going to put on a quarter inch of spackle, but I was hoping someone knew of some light weight foamy type filler. I will probably use your idea of carving foam, but I'm not a great hand at carving.
(Bondo, yes...Ive done autobody work since '79)
Yes, I didnt use the img tags, cause auto photos belong on auto sites. I can make anything look like anything given enough effort, but I am totally unfamiliar with foam. I tried wet sanding it with 220 grit and 400 grit paper...indifferent results. didn't accomplish much!
Of course on cars, I wasnt worried about weight!