This firm has some interesting fasteners. I found them when I began experimenting with tarps. I believe they work with silnylon, but you have to place them so a seam stops the edge, otherwise they can slip off.

http://www.shelter-systems.com/tarp-kit.html

I wouldn't suggest these clips for inclusion in a PSK -- too bulky and single-purposed. The pebbles should work fine. You can also gather a small loop in the corner and use that as part of a sheet bend knot.

I've never had any problems with grommets, but you could also consider sewing some small grossgrain tabs at the corners.