If Dr. S doesn’t mind me chiming in with an answer here while he’s off at the photomat - 'cos I do have an interest…
The reason for using a form, rather than sanding away everything, is so you can make more than one if it’s successful and cheaply/easily adapt the beast if you want to play with the shape some. It’s a male mold.
Glass cloth is not only cheap, but it’s not terribly flexible when glassed with polyester resin. That’s a good thing, as it lets you grind away some to tune the flex of the beast. 'Cos no way will it be right, right out of the box. You need to be able to play with it.
A few other things -
Epoxy is not only at least twice the price of polyester, it varies a lot in inherent flexibility - and thank you Patrick for making those test pieces. Patrick found that System Three SB112 epoxy was- what, lets say 2-3 times as stiff as RAKA epoxy, all other things being identical. Polyester is as stiff or stiffer than the SB112, though it tends to crack and fail when flexed beyond a certain point while the epoxy stands up to that degree of flex and returns.
Foams- again, polyurethane foam can be shaped easily and is compatible with polyester resin. Polystyrene foam isn’t, so that if you are glassing with polyester or making a male mold - especially a more permanent male mold with a glassed skin - it’s gonna be problematic.
For foam in the board itself, the sprayable or 2-part foams are useful. If you can get hold of them, sections of polyurethane foam sheet might be even better, laminated on. I am given to understand ( Via Dale Solomonsen) that sign foams are good for this, as well as some marine foam sheets which may be higher density. Nice thing about that is the stuff is more homogeneous than the poured/sprayed foams, so you will have less worries about odd points in the flex/stiffness of the foam.
And even better ( thanks MTB) are the foams sold for aircraft homebuilding, notably Divinycell - see http://www.aircraftspruce.com/menus/cm/foam.html for more. Also http://www.fiberglasssupply.com/Product_Catalog/Core_Materials/core_materials.html which has a good deal of Divinycell info which is applicable to other foams in some degree.
I have played with the idea of using balsa-core, myself, just 'cos I am kind of a wood freak, but question it’s use with regard to long-term adhesion in a highly flexed structure, besides the density of the stuff vs necessary buoyancy vs stiffness vs thickness. I don’t know that I would necessarily trust a flexible structure made with a significant amount of balsa-core.
Dr. S. has done a very clever thing with the masking tape and wax: ‘poor man’s mold release’ indeed, I’d call it clever man’s mold release. Nice thing about that is the foam is relatively accessible for modification, rather than having to rip off a layer or two of glass. .
If I may make a small suggestion, for the next one you might want to try brushing on a relatively thick layer of plain home canning/jelly making type paraffin- shades of hot-waxing back in the day- to deal with the tape edges. Deal with any brush marks with a hair dryer or heat gun…'course, you prolly already thought of this one…
hope that’s of use