I've seen some of those tanks with the plastic tubes cause a ton of trouble. They typically come with Phoenix ARF's. The airframes are great but the tanks and other hardware can sometimes be problematic. I would try t make a Sullivan or Du-bro tank fit. You may have to make some bracing from balsa or stuff it in there with foam. You'll never have a problem again though.

Another, possibly easier option might be to find a similar size and shape ARF tank from another manufacturer with a more traditional stopper. This might be easier to make fit. You could also try just buying a stopper and tubes at the hobby shop and see if it fits your existing tanks.