I also made some new Kep's 21 rear sponsons this past weekend. They are the exact ones that are included in the latest Kep's 21 RACE2010 rigger drawings. They are foam cored (Divinycell H60) and sheeted with 0.8 mm thin aero-birch plywood (CA glued with ZAP green and kicker). The 7 mm wide step on the bottom is 1.5 mm thick. They where also prepped with one coat of Z-POXY finishing resin, sanded and clear coated with Spraymax 2K clear coat. I will add some fluo Trim Sheet Monokote to the sides a little later.
Here you can see the simple paint stands I use. Just mount the prepped sponsons on the area where they will be mounted on the boat with some double sided tape. After they have cured its just to remove them and mount them on the boat.
The total weight each is about 14g - clear coated and RTR.