Once you are in the States and have possession of the un-capped magazines (i.e. 17rd pistol, 30rd AR15, 12rd shotgun), then after you shoot at Minot (for instance) you have to rivet the mags before you cross the border. Or, as you say, leave it there in a deposit box or at a buddy's place.
Sometimes here in Canada when you buy an AR15 PMAG 5/30 they simply rivet it. So you can buy it here, take it with you cross border, drill out the rivet, use it, rivet it, bring it back home. Really the AR15 mag at 30 or 40 rd capacity is the only important one to have there. If you are using your standard Canada-legal pistol and shotgun mags you are only at a small disadvantage. But Canadian-legal AR15 mags are a huge disadvantage. FOrtunately those are the cheapest and easiest to have duplicates of.