Bed Box questions

The time has come to move on from my plywood bed box. It's served me well even in my old Colorado. I'm planning on fabbing my own again, this time with Aluminum quick frame and HDPE. My questions for TW are:

1) HDPE or MDF? I'd like to make it as "water-resistant" as possible. I keep a softopper on my truck pretty much year-round, but water does seep in. I think MDF would be lighter than HDPE, but less "weatherproof."

2) Quick-frame or t-slot? Quick frame is cheaper, but t-slot is stronger I believe. Also more cumbersome to assemble. I'm not planning on really putting significant weight on this box, nothing more than some supplies from time-to-time. I did have a heavy generator on my existing box, but that was one time in 4 years..

3) Any suggestions on attaching HDPE (or MDF) to the aluminum flanges on the quick frame? (Example: https://8020.net/9015-black-fb.html)

4) Also planning on attaching rubber mat to the final surface, any suggestions on adhesive for rubber to HDPE (or MDF..)

5) Any takers in Louisiana/Mississippi/East Texas for purchasing my old box? Works great, just a tad heavy to keep in my truck, and I don't have a good storage option currently in my overflowing garage..

 

MDF will absorb water, HDPE will not.

 

Thoughts on MDF + Primer + Weatherproof paint?