i bought a hard polymer one from brasscatchers.com(e&l manufacturing), it is the ar flat top version.
i wouldn't reccomend it, as the mounting pins need to be locknutted to your picatinny rail (not quick detach), and they stick out about 1-1/2" from the side of your rifle.
3bucc.com sells a nylon bag type that has the mounting pins attached to the bag, so all that is attached to the rifle when the bag isn't in use is a small mounting block with pin holes in it, i will be buying one of these soon.