We purchased a platform from swimplatforms last year and love It! We are very happy with how It fit and how much easier It Is to board now!
If I would make one myself I would have laminated sheets of Hi-density foam then glassed and painted. With epoxy, glass and foam i would have had at least 3 bill's into just materials.
Anyway all of the crew are much happier now not having to use the finger mashing ladder the boat came with.