Some details around my setup.
1/2" (i think) plywood floor, which bolts down to 3 large bolts in the back for the factory seat mounts. Used 2x4's to support the middle and front and some extra cuts of plywood.
The carpet is cheapo outdoor stuff from home depot and is stapled down so i can easily cut a new piece and replace when it gets destroyed by the dog. I also cut a piece of wood and carpeted it for the rear wall, which will soon be bolted to some custom 3d printed brackets that wrap around some factory hooks on the back wall which held the factory seat in place. Back wall is also mounted to the floor with some L brackets, and i placed a few on the floor itself to serve as a backstop for when the wall is perfectly level up and down.
The dog crate was initially 2x3 with no roof, but this was tight for my dog and only good for short trips. I am planning for a cross country trip which will take 3 weeks and he will need to be able to lay down and get comfortable, right now i think its 3x3, and i have a roof cut for it as well, and finishing up the door just need to weld and grind some sharp edges down.
You can get the hog panel pretty cheap at tractor supply, its 25$ for a 16 foot piece (bend them in a U shape to fit in your pickup bed). Then use a grinder to cut panels for however big of a cage you want. The hog panels are tall enough for my dog to sit down and not have hit head hit the cage, but it's close. He's a 90lb Gshep so unless you have a bigger dog this will work fine. There are other types of similar panel there you can choose from which is taller if you have a bigger dog.
Crate is held down with some 3d printed brackets.