Thermostat/ Heat Reg with Rack System HELP!

I am building a rack system with tubs. I am planning on using heat tape to heat them. I have a thermostat already, but it only has 1 heat probe.

How do I regulate the tubs on multiple shelves? Should I just regulate each shelf with a thermostat, or should I regulate each tub individually and how would I go about doing either of these options? PLEASE HELP!

run one piece of flexwatt in an "S" from the top level to the bottom level and plug it into your thermostat

That's a very clever setup. Unfortunately the setup I'm working with with wood with a bookshelf type build, so it's kind of hard to snake the flexwatt. I'll probably have to use strips. I think it would allow for easier temp reading and regulation too.