As JLC mentioned you can order heat tape from reptile basics. There is an option when ordering for them for them to finish off the ends for you.
Just measure the length of each shelf and add enough to reach down to the next shelf below. My 28 quart, 7 tall racks used 18 feet worth.
Id recommend 3" or 4". Just weave and use silver aluminum tape to hold it down. Your tubs should fit just fine!
EDIT: I didn't realize it was a closed rack... You may need to have someone help you with it. Put a small hole in the back corner of each shelf in order to wire individual heat tape pieces...
You may definitely need some help if your not that handy.
A lot of keepers loose their collections due to fire... Probably unsafe wiring...