I'm looking for the "best" handguard for my next build which is a 10.5" 300BLK upper integrally suppressed and I need a handguard to cover it, railed or smooth doesn't matter but needs to have a inner diameter greater than 1.5" any suggestions?

Look into:  http://retailbuild1.blackdawnguns.com/?product=mmr-free-float-foreend

I used them for a piston build that required a 1.5" clearance.  That dimension is not listed on their website, so double check with them.

Spikes/seekins BAR, seekins mcsr, pri tube, apex tube, rainier evo might work  and KAC URX4. Some others just ones I could think of I know Samson makes some.

i've begun an integrated suppressor build myself, just started on the lower but i'll be using the yankee hill smooth free float rail on mine, its got a 1.75 in diameter. I'll also be running the AAC 762 SDN 6 (1.5 inch diameter) just as long as the ratchet mechanism doesn't get in the way, which is something to consider if you want just a thread on suppressor or QD, still waiting for AAC to email me back about it. heres the rail
http://yhm.net/accessories/forearms/smooth-freefloat-ar-15.html

Aero precision, same as seekins afaik 1.8 id