I build this board out of Home Depot and Lowes extruded polystyrene (XPS) home insulation which is easily accessible virtually anywhere.
This is just how I built mine, and it was specifically designed with fly fishing for carp on the flats. If you are thinking of making your own stand up paddle board and like the concept of a standamaran you should customize it to your own wants and needs! The most likely change you may want to make is to make the pontoons longer for improved speed and tracking. Additionally, if you are much heavier than I am (~175 to 185lbs) you will need to make the pontoons longer in order to add buoyancy.
Single-click to enlarge any of the drawings or pictures. Feel free to download them for personal non-commercial use but do not re-publish them without written permission
If you do decide to build your own board of any kind, be sure and check out swalylocks for general advice and surfersteve for the first time builder.
PHASE1: Building and Shaping The Foam Blank:
My standamaran was built using methods similar to surf-board and traditional SUP manufacturing. The process starts with a shaped foam blank which provides the shape of the board and provides floatation. This blank was built out of Home Depot XPS foam which is virtually impermeable to water.
The foam is then covered in a thin layer of fiberglass which provides strength and durability. Because I used XPS foam the fiberglass is applied using epoxy resin which will not eat the polystyrene foam.
I added white pigment to the epoxy to avoid heating the blank and causing de-lamination due to expansion of trapped air. XPS has more risk of de-lamination due to heat than other foams because it is non-porous to the point where air cannot migrate through the blank making the addition of vents ineffective..
UPDATE: January 2016: I have now probably spent something on the order of 100 carp on it, including my biggest Colorado Carp on the Fly to date. This thing is a blast to fish from.
What it absolutely rocks at: This design is extraordinarily stable. I have never once, even for a second, been remotely close to falling off sideways while standing up. I have, however fallen off forward/backward twice. Once when I was still getting used to it I leaned too far back and it skated forward out from underneath me and I did the Nestea plunge in about 3' of water. The other time I was poling it backwards out of a small inlet and bottomed out on a 4" deep sandbar. Once again, I went over backwards!
The fiberglass schedule detailed above is pretty robust. Probably 50% of the time I have spent on this thing has been in flooded timber and brush. As you can imagine that has led to a significant amount of abuse - but I have had no significant durability issues beyond a few new small holes in the fiberglass which I have not even bothered to fix and does not ruin the SUP because the Extruded Polystyrene (XPS) core is virtually impervious to water.
What it has issues with: This is a great stand-up fly fishing platform but isn't necessarily very fast. I usually treat it like a sit-on-top kayak when going from point A to point B. Even when I am able to continously paddle sitting down it is a little slower than a beginner level SUP designed more for stability than performance. Once again, if you want a faster SUP, make the pontoons a foot or two longer!