We've tried all these things believe me. The system we have right now is the cheapest, most direct and most effective way to do this. The cold also spreads throughout the board btw, including on the underside.
The only viable self contained solution is a cascade systems and those have their own costs, so you're looking at over R10,000 minimum and that will not give you Dry Ice temps which costs you max R500 a session.
If you're super serious about going Extreme, you need ideally two pots, a dewar, flasks, temp probes, sealing spray/art eraser/ armorflex and shop towel. and of course the LN2 from Afrox which is going to cost you minimum R3K per session.
It's an expensive hobby, but if you just go DICE, you can save on the Dewar and LN2 obviously and pay maybe R800 per session in total (acetone + dice)