You could also go and use the HERA Arms stock conversion, adds a UMP folding stock and nice grip to the USC trigger group and receiver (no mods at all). Keep in mind your 922r compliance parts, you would need IIRC 3 US made parts to comply...
Otherwise, on a full USC to UMP conversion I've heard to guys using little stick-on chair "pads" (used to protect a wood or tile floor from chair legs scratching it) to act as spacers on the mags. They need to be 1/8th of an inch thick on each side, but are supposedly completely hidden and non permanent.
Something like these:
Furniture Glides Self Adhesive, 3/4" # FE-50111 by Mintcraft
not sure if those will work, but you should be able to find them at your local hardware store no problem.