Where to buy...?
 

Nothing fancy. Pretty simple.

But where can I find pure white backing for my tank?

You can spray paint some thin pexi from Home Depot.

White corrugated plastic sign material, most of the big reno stores have it

Rona has white ceiling light panels of various types. They have flat white plastic sheets about 1/8" thick in 2' x 2' and 2' x 4' sizes which I think would work well.

Ed