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