One other way to make tempory testing Model Floats and hulls, is to have a stiff spar, say of Carbon? Grp tube or a small section of Hardwood, say 18 X 25 which is the support along the top side of a White Foam Float (Polystyreene Foam). This Foam, as used in packaging, is not good against impacts, however for testing different shaped hulls, they can easily be made by carving and sanding, covering again with the infamous Cling Film, and attached to the Spine. These Spaines can be secured easily to Beams, and within a few minutes try a design out. It costs very little if you buy a sheet of it from a DIY Store.
There are two sorts of this Foam I have come across. One is tighter smaller cells and the other larger. The Larger cells that make up the Foam tend to 'break-out' as you sand it. However, by using a coarse grade of Paper, you can sand it down using light brushing strokes. The best is the other closer cell Foam, which you can get some really good finishes on.
When you have decided on a particular shape to your Hull/Float, you can then Divide the lenght up equally, cut through at these stations, and that will give you a profile of that Station. Using these Profiles you can then make Bulkheads say for a Balsa Model or for a GRP Plug.