One can epoxy a thin steel plate on the front of the mag body which has a hole of correct diameter drilled into it. This works for Thermold, Gen 1 Tapco, and most any plastic mag intended for the Min-14/-30
The Gen 2 Tapco had a spring steel "U" shaped collar which reinforced the mag body (and front locating hole) at this point, so the principle is the same, just different execution.
I've done this, as an experiment. It worked.
BTW, Ruger is usually the most expensive place to buy Ruger mags. Always shop around!