Washed Ashore--A 4'x8'x12" styrofoam block
 

Thardware stores sell adhesives for foam. I stuck foam insulation on the
concrete basement walls, then sheets of drywall over top with the stuff,
back in 1980. Still sticking to the walls. Applied with a caulking gun.

Washed Ashore--A 4'x8'x12" styrofoam block
 

how about a sheet of plywood glued to each side. Cut about 2" off each
side so the edge of the foam can act as a bumper Then use threaded rod
to through bolt your cleats. The bolts would also keep everything
together if the glue failed.

Thardware stores sell adhesives for foam. I stuck foam insulation on the
concrete basement walls, then sheets of drywall over top with the stuff,
back in 1980. Still sticking to the walls. Applied with a caulking gun.

Washed Ashore--A 4'x8'x12" styrofoam block
 

Looks like you got plenty of advise so here is more. I think I would cut the
block into two pieces, each 2'x8'x12" and make the raft something like 6
feet wide, making it maybe a 6' X 8' deck. I picture myself on a 4x8 deck
that is very tippy. Also maybe a combing board around the periphery to catch
my tools and stuff when they try to slid off ?

--
Ron White
Boat building web address is
www.concentric.net/~knotreel