Besides the NDS receivers the options include unbent receiver flats, but these require jigs and a pretty stout hydraulic press to bend.
If you do flats, make sure to get properly hardened ones, otherwise the holes can wallow out in record time. The previously mentioned Maadi and unmarked AK as examples.
The AK can be worked on by your average owner, replacing the grip, handguard and such but the front and rear trunions as well as the barrel are beyond most owners.
One thing to remember about the underfolder models, the stock is a direct copy of the German MP38 /40 and wasn’t up for the mild beating it received from the 9mm let alone from the x39 round. The side folders are the way to go unless you like a stock that gets floppy, very floppy.
To this day, the best AK variant folding stocks were on the Valmet and Galils. The Galil stocks are nothing but a copy of the folder from the FN FAL so its built stringer than it needs to be for an AK.