Best way to waterproof MDF sub box??

I have done some searches on this and see that Resin, painting and truck bed liners are all popular choices.

I have never worked with resin and not sure what to expect. How fast does it set up? Any application tips?

I want this to be sealed really well so I plan on doing multiple steps. But not sure what order is best.

I bought a bottle of resin and some Rust-oleum marine primer for wood and fiberglass. Should I prime it first or use the resin first? If I were to do the bedliner what step should it be?

Do I do the inside AND outside of the box (why the inside?)

I plan on carpeting it as well and it is going in a well covered spot, but I would like it as waterproof as I can get it.

Thanks