Shove the mag in your belt. Don't try too hard to get it in there, or you're just wasting time. As long as the scenario doesn't call for that mag to be reused, it's OK if it falls out during any movement that follows the reload. Very few scenarios will need it again, especially with the 18 round limit.
It almost never falls out of my belt, but if it does, I've made an effort to secure it after the reload, so I shouldn't be penalized.
The "tactical gods" would hate me for typing this...much less thinking it.