I had trouble getting @unhappyarms:c 's Cetme Mag to fit and sanding wasn't enough, I had to force it in and then pulling it out actually ripped the feed lips off. I used the STEP file he provided to bring in the rear of the feed lips and the top of the forward latch a bit. I highly recommend trying his original version first, as these changes definitely compromise the integrity of the feed lips.
Cetme fat mag
Sten Mag Mini Bic Sleeve
rubber band mag replaceable lips