Seeking recommendation for water resistant enclosure coating

[ColumbusJBR]

So I've got a 2009 Honda Ridgeline, and under the back seats (bench) lives a ported enclosure I built about 10 years ago that housed 2 DC Audio M3 8" subs.  It was constructed out of 3/4" MDF. I finished it w/ rattle can Rustoleum textured spray paint.

 

Anyway, the box is kinda falling apart due to moisture.  People / kids get in the back with wet shoes, occasional rain when getting in and out, etc... So some water accumulates at the bottom and has caused the 3/4" MDF to swell up and start coming apart at the seams.  A few years ago I patched it up (poorly), but now it's time to rebuild; she's lived a good life.

 

If anyone has recommendations for a more water resistant type of finish (or different build materials?) I'd greatly appreciate it.

 

Thanks!

[Joe X]

You can use marine grade plywood and use phenolic glue to build the box.

[EricM9104]

You can coat it with 2-3 coats of exterior grade polyurethane on the new box. Could also use a rhino liner coating. As well as using the suggestions from @Joe X