I screwed 1/4" starboard strips to the bottom of the coffin and let the tank sit on top with no adhesive. (Tank is secured by 4 mounting flanges on top.)

I used strips of these. They seemed to stick to tank pretty well.
https://www.tankandbarrel.com/mounti...ll-p-3858.html

I like your plan to add bulkheads. My boat is somewhat different than yours but I did the same as what it sounds like you’re planning.