Utterly Mad

I'm guessing you have a Mossberg based on the description (ok, so it's a total guess). Having no idea whatsoever as to what sort of fabricating skills you have, I can't really speak to if your fix will work. Given how simple the factory mags are, and that you're a clever dragon, you should be able to make it work at least "ok".

The only place I can forsee a ton of trouble is in re-sizing the spring. The pressure required to reliably feed anything relying on blowback is a bit of a fixed commodity. Too little pressure and it won't feed. Too much pressure and it will cause the action to bind. Err on the side of "way too much", as it's only a 25 round spring.

Worse come to worse, I could probably walk you through how to make a working 5 round mag out of it.

...yep, a Mossberg 702. The spring is, well...actually two springs separated by a little plastic guide thing, and both seem the same general dimensions as expected of a 10-round magazine.

well question is whether sheet metal malleable enough to bend in place with various hand tools will stay on if the spring is compressed. I keep forgetting I should at least put rounds in the mag to test it.