Rookie in the house. Challenging the assumption that there are no stupid questions...

I'm looking to build my first box for under the rear seat of an f150 and have a question about a design I plan to emulate.

The box is about 5" in height at the rear and around 8.5" in height at the front, due to the slanted nature of the rear bench seat. The design I've found uses two separate pieces on top (running lengthwise) to accommodate the change in height. The total depth is just over 14". Basically, one piece about 6.5" wide and another almost 8" meet in the middle where the 8" piece then angles up.

My question is: how do you join the two pieces together? Are they each cut at a 45 degree angle and then glued together? Is there another way to enhance rigidity ? (lol) Meaning, I don't think it's possible to screw it there.

Thanks in advance,