I've built a few of these with a few friends. If you can, find some roof rubber membrane, asphalt-type roofing or even metal roofing or decking to get a cap on that thing. You'd be amazed how fast that water will wear on those flat horizontal surfaces....even with that pitch you have.

It does look out of plumb and level, but it's the way the pic was taken across the incline. If you tilt your head to match the image...it comes out pretty close. Nice work with your ice shanty on stilts.

Good luck.