I watched Sgt. Rock's excellent video on the cuben tarp construction, which uses flat seams. I'd like to make one with cat sides.

I'm concerned about bunching when hemming. When I've made sil tarps, any minor wrinkles were hidden under the grosgrain trim. Looks like no such luck with folding over the cuben.

Is there a way of hemming along the curve smoothly? I'm hoping I can just press it tightly as I go and generally be careful, but I want to be sure. I'm picky and it's not cheap stuff.