my favorite thing to use is this speedo kickboard deal. it has channels on teh bottom and really works well. its flexible and makes bodysurfing alot of fun. to make one, get some foam and shape it to the shape you want. lapping it would probably be a problem so you could just glass one side at a time, then put rovings all around the edge, kinda like you glass a fin. then sand it all smooth and go for a body whomp.
another alternative is a paipo board. piece of plywood with slightly rounded nose and no fin. use it like a boogie board except go faster and hold a higher line. just varnish it and wax it up. 1/2" ply works the best im told, the rails hold in really well as long as your in the steep part of the wave.