Tubs can be either in a rack or on a shelf.
Extension cord and extra wire for multiple tubs if needed.
Flexwatt UTH pads wired and plugged into a t-stat.
Locking tubs if using a shelf set up.
This is my current set up with 28qt tub melamine rack, 6qt tub melamine baby rack up top and melamine 5 shelf for larger tubs off to the right.
There is 11x12 flexwatt on each shelf.
It's really simple and cost effective and no worries about humidity and all the other issues a tank brings to the table.