3/8" Baltic birch plywood with 1 layer of 6 oz. fiberglass on each side laminated with UV poly resin (had some extra quarts from doing surfboard repairs). I drilled and filled all insert holes with 5 minute epoxy. I used t-nuts for the inserts.
The board turned out better than expected and has worked well in winds up to about 15kts. I've been riding it for about 8 months now with no problems. It has no rocker at all so it sucks in chop and I didn't thin it out at all other than the rails so it's stiffer and much heavier than it needs to be.
I traced the outline from a friends Slingshot Glide, so I'm guessing the dimesions are around 150 x 46 (it grew a little with the solid glass rails).
Lessons Learned:
1. Board is too heavy - I could easily thin out a lot of volume just outside of the pads and decrease the weight quite a bit. Adding the solid glass rails and doing the hand layup added significantly to the weight as well. After the hotcoat I sanded off nearly 1 lb. of weight!
2. Some rocker is nice - This board works really well in light winds and gets you going and stays upwind but dead flat rocker catches the rail a lot and combined with the weight gives your legs quite a workout.
I'm working on board number 2 now. It's going to be a 138x44. Same 3/8" plywood core, but I'd like to add ABS rails and vacuum-bag it to a rocker table. I'm going to have the core CNC cut so I should be able to dial in the flex and weight a bit better overall.