SLR finally released their billet AR grip adapter. I ordered on 11/8, it shipped 11/8 and I had it in hand yesterday. First impressions: The machining and finish are very nice. I got a slight chuckle though that they're currently shipping them in the bags for their muzzle brakes. That said though they decided keeping the finish nice during shipping mattered- A+. The first hiccup though came when I realized the adapter doesn't ship with a screw, and the extremely long one obviously won't work. I emailed SLR and had a reply from Todd extremely quick with the necessary info and I believe he's updating the website to reflect it.
Screw needed for installation: M6-1x40mm*
*I went to Jerry's here locally- think Home Depot but a little better selection on some things and a local place. The longest M6 screw they had was 30mm. It appears to me that there is plenty of thread engagement here.