VL boot "boxing" question

[87RB30VL]

Ok so I mounted my Amp to the back support of the rear seats this arvo, and I want to make / cut a template on Plywood to attach to the back of the boot similar to the attached picture (see below), but I am unsure if I can just cut the section I want, or if I have to fully "box" the boot to hold it all together. I'll try to explain it in words, then in pictures. I want to fix a piece of ply on the floor of the boot (just to make it level and look cleaner), but add a piece on the back, like a _| shape so that the | sits at the back of the boot (covering my amp and housing my sub), and the _ just to make the boot look cleaner and not all bumpy due to the **** soundproofing under the carpet.

Circled in red is what I want to do regarding the back piece and the floor piece, but I don't want to cover the spare wheel or the opposite space (scribbled in black):

Here's my boot situation at the moment (I won't have the bulky sub box as the sub will be removed and mounted to the rear ply panel):

I guess what I'm trying to ask, is how can I attach the wood to the base of the boot without risking piercing something like the fuel tank etc, and how can I attach it so it can support the Sub Speaker on the backing piece (as per picture 1) but won't slide around, or collapse when braking, turning corners etc.

Sorry if this is the wrong section, I just thought it'd be easier to ask in the VL section rather than audio since it isn't really about the audio side, more the DIY side.

 

[drewVHSS]

Take your tank out prior. It's pretty hard to hit much else when drilling thru the floorpan with the tank already dropped. (doesn't take long if you've run it fairly low on fuel prior)
Try to mount base board by screwing inline with the chassis rails (far less risk of hitting anything other than a cavity space)
You'll probably find the front face for the sub box is either a beauty board and the main box is bolted down through the diff hump section of the floorpan and also bolted to the panel behind the seat. Or the front board for the sub box is butt mounted to the rest of the box hence being ontop and overhanging to cover up the hinge space.

It'll probably be easier for you to make each section one piece at a time using cardboard. trim to size and shape and then cut out mdf to piece it together bit by bit.
Don't forget you'll need smaller square blocks in each join/corner for mounting each piece together.
Liquid nails will be "your friend" along the way as well as your cordless drill.

Edit: dont forget to swear alot along the way because you only measured once and had to cut twice! If your real estate sends a letter about complaints from your neighbours (if renting) then you're going about it the right way!!!