This is the polymer Sten pattern mag that is supposed to feed reliably, but I figured out it won’t work fit in my VMAC9 unless I modify it.

MPA says it fits in their newer guns without needing fitting.
https://masterpiecearms.com/shop/9mm-polymer-magazine/

https://imgur.com/a/TlMYw9z

Price is $30 in Fairfax or plus shipping.