Without hesitation, I went to borrow a jigsaw, bought some hinges and pulled out a measuring tape. The DIY lasted about two hours, the material was much tougher than we'd thought.
Taking the initial measurements and trying to maximise the use of material.
Cutting of the raw material.
Taking the final measurements and making the mounts for my sub.
Finally, after screwing everything together and making a lid so that I could access my spare tire without removing the sub, this is the final outcome.
Tommy holding up part of the board. Crystal clear as you can see.
Here's a contrasting pic with a slight shadow of the material.