By the way, thank you very much on the mytarp.com suggestion. They're selling the same 18 oz vinyl fabric as most places at literally half the cost: http://www.mytarp.com/18-oz-vinyl-coated-polyester-fabric-61-inch-by-the-yard-roll.aspx vs. http://www.sailrite.com/Shelter-Rite-18oz-Yellow-61

Between mytarp.com and dxengeering.com for the tubes, this boat is going to be 30% cheaper than I was planning!

I'll post another thread later about this later on, but I'm going to try 3D printing the cross sections instead of buying a large sheet of HDPE. It should make things much cheaper. Thanks for your suggestions!