I have an alternative for you. Check out snsindustries.com, they make a Colt style 2 piece block for Sten mags that works with Colt or Oly systems.
You will have to cut a slot in your mags for it to work with the standard AR mag release.
The nice thing about this system is that all of your IA drills remain the same, regardless of the caliber of the upper. As well, if you order the block with the ejector slot then if you ever switch to a Colt/RRA type of 9mm then all you have to do is install the ejector and you're ready to rock.
It is also one of the cheapest mag well adapters out on the market. I think that I paid $68 US shipped (w/ ejector slot, in the white). It looks to be well made but I never ended up using it because I couldn't find any cheap sten mags to try it out with.