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.