just to confirm what builzoid and der8auer said, to make a ln2 pot you have to use two rules:
In case of copper base
1) (°C/s) = [base weight (g)]*[0,385 (J/(g*°C))]/[load (W)]
2) delta temp (°C or K) = [base height (m)]*[load (W)]/( [390 (W/(m*K)]*[base area (m^2)] )
so you have to know on what CPU you'll use the pot, eventual CB
for example a not-so-height = (not-so-weight) base gives you a light base, fast pot