Definitely building your own is the way to go IMO.

This is a true cooler I picked up for this season.
I'm going to have $425 into it and it will hold 18 6qt tubs with the current amount of shelves and can be expanded further with another shelf.

Herpstat thermostat, 60 inches of flexwatt heat tape, a bathroom ceiling fan wired to a dimmer switch, water bottles for top and bottom to keep stable temperatures.

Simple to build and larger and "better" than anything you can buy for the money.