Put enough pins through the crack to hold it together.  Saw the butt stock off at some convenient spot, the reassemble with two door hinges with removable pins, one hinge on each side of the stock.  Done right, the stock can be folded either direction!

Can't say I have an equally brilliant idea for an under folder.

For serious answer (although I'm not completely joking above), search the internet until you find a candidate.  Maybe look through Brownell's and the companies that sell conversion stocks for shotguns.