When designing a pot you want to achieve the following :
1 - temperature controlable to a degree
2 - no temperature swing
3 - keeping load, minimal temp dropping
---->enough mass presence in base that can be touch on lot's of places by the LN2/dryice
How to reach this? Wel it's tricky and i recommend you to make a customload tester. And see what it does when having a different structure base.
By my opinion the best design yet to date is team Japans design, and it's pretty simply a base with drilled holes.
ps, go square