I have a bunch of Thermold 30 rounders for the Mini, they seem to work alright.
3 for free!, I mean $25
https://www.sportsmansguide.com/product/index/thermold-ruger-mini-14-magazine-556-nato-223-rem-30-rounds-3-pack?a=1174807
The factory 30 round ones are for SHTF!

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!

When I had mine, Thermolds were pretty good. Ruger 20’s too  but  those were hard to find.

I have ruger 20rd mags that work great. I have several tapco gen 2 30rd mags that work very well also. I even have a promag 20rd and a 10rd that work well also. I even scored a few of those cdnn original equipment straight 20rd mags a while back that work well.

Gregcote llc had 30 round factory mags last week, I bought 2