I've got a set of coro floats. The aircore variety. Dimensions of similar sized floats are on my site.
The ac floats are 40-60 sized, but, they weigh about 20 oz, and thats using mostly 2 mil. The easiest, lightest way to make floats that i know of is to just use 2 or 3 pieces of 1" or 1 1/2" foam, and cut them out to the shape you want, glue em together, and either cover them with sign vinyl, or ultracote. Whatever it takes to waterproof them.
They should be considerably lighter than the coro floats, and alot less work to seal them up. Next set i build, will be foam and sign vinyl, the coro floats are just too heavy to suit my tastes.