HCA: Hermit Crab Association

I am buying a new UTH for the back of our terrarium. What size should I get?
The terrarium is 24" W x 18 " D x 12" H

I live in Canada, so am unable to order the ultratherm heaters as shipping is ridiculous.

Available options:
Exo Terra brand 11" x 17". Or 10" x 11"
Zoomed brand 8" x 12" or 8" x 18"

Any recommendations are appreciated

I'd do the largest Exoterra you can fit. Double check the instructions or with the company, but I think their Heat Wave series can be insulated.

Any chance you can get Flukers? I use those and they are excellent.