I have a couple of MP5 variant .40 guns and have been searching for parts and accessories for years and have never seen any mags other than 30 rounders. The few "clone" models that I am aware of were all 30 as well.
Make some type of plunger or block that will fit inside the mag under the follower that limits only 10 rounds loaded. The best I remember there is a void in the middle of the spring that would allow for a dowel and hold it in place. Basic same principal is a plug in a semi auto shotgun.
A forum dedicated to and laser-focused on Heckler & Koch firearms, Heckler & Koch accessories and the owners and enthusiasts that love them! Come join the discussion regarding HK pistols, long arms, NFA arms, HK accessories, HK history and trivia and more.