It depends on what leakage you are prepared to put up with. The only tank I have found that is genuinely leak-proof is the Durst. The drawback is that, as I understand it, they only made a 120 and 135 tank and each only takes one film at a time. I don't think Durst made multiple film tanks like Paterson and Jobo

If I may ask, what's the problem with very small leaks and by that I mean leaks that after the normal development time and n inversion agitation have not resulted in the critical loss of developer to the extent that the top of the film is exposed without total coverage by developer.

A few extra cc's of developer and fixer at the start will ensure that the film is always covered.