A Bog Deathgrip tripod came up for sale locally and I thought about doing something like this. But as is typical of me, I thought about it too much and the tripod sold quickly.
I just found two, four screw rings and they fit in three slots of a picatinny riser with plenty of space in between. But I'll be interested to hear if two works for your Impact. I just looked at an image of an M3 and it appears there wouldn't be any interference with the trigger guard or bottle. Of course you could always use a picatinny riser/extender to space them were you wanted and just leave that rail on your M3.
You're making me regret not buying the one locally. At the time Dvor had them on sale for the same price but at the time I wasn't aware there were so many models and that the local one may have actually been a great deal.