I built my own J racks and I think my basic design would work very well for you.  I cut the profile of the "J" out of some spare 1/2 ply, two pieces, then sandwiched a 2x4 as a back bone.  the profile was actually custom tailored to exactly match the profile of the hull, hence you end up with a flat back (from the 2x4) and two raised ridges that the hull rests on (the plywood) that I glued some foam on.  The corner at the bottom of the J is were the two 2x4s meet and the ply creates the connection, so each J consisted of 2 2x4, 2 pieces of ply and I used deck screws to hold it together, expoxing the sandwich once I got it just right.  Just returned from a 2,400KM trip to Tadoussac in Quebec with both boats, worked like a charm.

happy to send pictures david at dragonsong dot ca