: Has anybody made one? Is it just an arc on the edges, or is there some
: special shape about it? Seems expensive to me, a couple hundred bucks for
: a chunk of ripstop nylon.

$20 for the nylon.
$180 for knowing how to cut and sew it!!

Most of the ones I've seen appear to have both a catenary cut at the edges, as well as a catenary to the center seam.

A catenary is the curve a rope or chain will naturally take when it is suspended by its ends. It is a very strong structural shape.

I think the Sydney Opera House is constructed of inverted catenary arches.

If you have good sewing skills, you could make a great tarp yourself for less than $40. I think the trick would be to suspend the fabric on a wall, and hang a piece of light chain from corner to corner, and trace along the chain to describe your catenary cut. Do the same at the edges, and hem well. Use plenty of webbing loops for guying flexibility, and bar-tack every seam very well.

Shawn