yea a 6 foot extension cord is like $1.88 at home depot. i use those for wiring my on/off thermostats too.

you could wire each piece of flex and plug them into a power strip. that way if you are not using a shelf you can just unplug it and just that tub won't heat. or if you have say a animal with different temp requirements you can plug that tub into a diff thermostat.

just some thoughts, i don't know what you are planning. so i hope this helps.