If it is only one or two of the cross-flow tubes, you can cut them in two with dikes, roll the ends over and pinch them shut, and using a propane torch or one of them little butane torches, solder it shut. If you don't have a torch, pinch them over a couple of times and use vise grips to get a good crimp, then put some sealer in the system and you should be good to go for a while. A good temporally sealer that 99% of all 5vers have is, coarse ground black pepper. It won't clog the system, easily flushed out, what doesn't get flushed out will dissolve, but plugs most leaks.