Perfect if you care about width more than adjustable length - SIG SAUER MCX/MPX Thin Side-Folding Stock Black - All NFA Rules Apply STKXFOLDTHINBLK

Installed onto my MPX carbine and it now fits perfectly into my bag. The folding mechanism takes a little getting used to (hint: push down with thumb north of hinge and pull up with other finger) but once you do it a few times it becomes send nature. Color-wise its a perfect match for the MPX.