Cool.....your fingerlings came in. I'm excited for you... ...I remember when I got my 1st ones too.

The dura-skrim I use is a 20 mill polyethyelene....3 layers with strings in between the layers. It certanily seems plenty strong enough, for raft set ups. Might work for gravel too, as long as the rocks werent sharp.
I order the roll thats just over 6' wide....works great in my troughs. Which are 4' wide and 1' deep...the little extra width comes up and over the top.

If you want to check it out...here's a link...
http://www.americover.com/20_mil_reinfo ... 4_prd1.htm