I have a 20" Merchant barrel that I want to use as the barrel on a SPR type upper.  I need to find a float tube that either has rails already or that I can attach rails to that won't be too expensive.  I'm considering the Phantom II from Fulton Armory.  I've also considering the FIRSH but that's getting a little close to "too expensive".  Anybody have opinions of either of these or any other float tubes that might fit the bill?

Thanks.