My Paterson System-4 tanks don't leak either. There are a lot of older Paterson tanks (with screw tops, white plastic lids etc) which are so old now that the original soft plastic seals are either hardened or missing - perhaps it is those that people have problems with? Anyway, the 'tupperware lid' style tanks work fine for me.

Inversion agitation also seems to give better agitation, if there is enough air volume in the tank to let it work. That could be tricky if the developer is highly diluted and you need a large volume to hold enough active dev, but possibly in that case one would be (somewhat optimistically) trying a variant of stand-development and using minimal agitation.