The problem is insufficient agitation.(posted 8663 days ago)Since you can't get a big air space in the inversion tank, I'd suggest you just run a tank line. Agitate by completely lifting the reel from the developer and plunging it back in, continuously for the first 30 seconds, then five seconds every 30 seconds or, probably more convenient, 10 seconds every minute.
Tupperware-style containers will be fine for this; no need for expensive stainless steel stuff.