The ones I welded together only held 17, but you are correct, even though they have always worked I would not trust my life to one; not on a dare.
People ask this question it seems once a week around here and there are no really good ways to convert the 10 round mags. Some people have taken out the inner plug and glued the base plate on, but you cannot take them apart to clean any more and what if the glue fails and dumps your load of ammo on the ground in front of you. How embarrassing to prematurely dump your load right there in front of God and everyone!
One should either live with the ten round mags which are dirt cheap as mentioned, or quit being a penny pincher and buy the full caps. Also as said, with ever full cap HK mag CDNN throws in a free 10 round mag. That should soften the blow.