I'm building an upper with a 16" 300 Blackout barrel with pistol length gas.  This isn't going to be a tacticool gun, just a range tool for testing loads, so I don't want to spend a bunch of money on a fancy rail.  I want to find a way to install a Magpul handguard on this barrel sort of in a "dissipator" style.  I'd appreciate any suggestions.  I'm not against a "cheap" clamp-on FSB, folding or otherwise, but I really want to make sure I get one that works.  I'll have the barrel in hand by the weekend, but I don't know the exact diameter of the barrel at the carbine gas port point.

I had to make my own. It has set screws, clean and tidy.
But others have used a spacer held on with the F.H.

If you are testing loads, then clamping stuff to the barrel may affect accuracy - how about a cheap free float............

http://www.leapers.com/prod_detail.php?mitem=mount&itemno=MTU005SS

if it will fit over your gas block

Per usual, shop around for best prices, especially during the Labor Day sales.

I decided to take nada's advice.  I'm looking for probably a midlength free float rail, and to be honest, I'll probably go a LOT cheaper than the Leapers rail nada linked to.  All I need it to do is give me something to rest the rifle on, so I've been looking at really cheap rails, in the neighborhood of $50.  As long as they actually support the gun, I won't care if they are "combat proven," or any of that stuff.