To be fair, this is how one manufacturer handled this by building a lower from the ground up with a fixed magwell in mind. Companies like JC Arms handles it a totally different way. That seems a lot less complicated.. Using an already manufactured lower like the OP has.
Oh, so you don't want to pin the mag for a max of 10 rounds. You're looking to permanently fix the mag in the lower to make it "MA Compliant".
I believe they make special mag non-release buttons for that.
Out of curiosity, If you're going to pin a 30 rnd mag to 10 rnds and make it permanent in the gun.. Why not just use a 10 round magazine? The 30 can get in the way of the table if you're bench shooting.
I think the size of those states and the ruralness of them contribute to the gun club locations.. You may have 10 gun clubs and 30 towns and 3 million people in that circle.. They 4 towns and 200k people..