[OJFord]

40 US gallons ~= 150l, seems (even a bit bigger to accommodate the different shape of the tub to fit in) perfectly within the realms of chest freezers, is getting a different freezer instead out of the question, i.e. to simply put the tub in the freezer?

If you do want to do it the way you suggest, probably better to run a closed loop as a heat exchanger in the tub to be cooled, rather than circulating that fluid directly. I'd have thought you'd also want the coil inside freezer to be submersed in something, even if just another tub of water (block of ice) to improve transfer, but that's just intuition/guesswork, not sure about that.