Two 55 litre plastic containers with lids. Cost under $10 each.
One for wetsuits, helmet, towels, hats, block-out, boots, etc.
One for fins (in a fin bag) + spares including wet'n'dry sandpaper, screwdrivers, screws + bolts for fins, gaffer tape, Selleys Aqua Epoxy Fix stuff, electrical tape (to put around sore knee joints), stainless steel washers, spare rope for uphauls, downhauls, inhauls, outhauls, a multi-tool that does about 20 different things, piece of 50x75mm wood about 20cm long for persuading things that won't come apart, two downhaul tools, 1 spare uni joint and base.
I think that might be about it.