This is how I built a false floor in my 05 WK. I wanted to hide my stereo amps and retain the cargo area. Its nothing special, can be made for under 50 bucks if you have the tools already.
2 sheets of 2'x4' MDF
3 2x4s 8'
1 roll carpet
1 can 3M spray glue
Wood decking screw(dry walls fine)
I removed the stock floor first. Just lift up the plastic trim piece around the cargo door latch and it pops right off. Remove the two nuts holding the floor down, then lift up the floor and pull it towards you, it slides right out because its only held in by metal tabs in the front. For the hight I used 1/2" pine ripped down along the top and bottom horizontal. Then on the top horizontal only I laid a 2x4 on top of the 1/2" pine board, that gives it a slope. Note I laid the framing over the stock soundproofing. (the stock floor slopes up at the top)
Then I ripped down some 2x4s for the verticals, that go front to back. The middle 2x4 I have not centered for my amps to fit between. The middle one is also slightly longer to support the MDF you will see in a following pic. One long screw down thru the vertical 2x4's into the horizontal boards below were enough to hold it together.
Then I used a full sheet of the 2'x4' MDF for the top section, ripped it on the side to fit. I used 3/4" MDF. I first used 1/2" but it wasnt strong enough and bent when I kneeled on it. I wrote the meaurements I used. For the lower section I traced it useing the stock floor as a template for the curves, then cut with a jig saw. I sanded all edges by hand for a nice edge.
Installing the carpet was easy, 3M spray glue on the top surface and edges of the MDF, wait till its tacky then placed the board over the back side of the carpet I had laid out on my livingroom floor. I used a small hand roller to get it on nice and tight. Then I sprayed the glue on the back side of the MDF around the edges, folded the carpet over and used small tack nails to temp hold it. Trim the excess carpet off.
Here you can see why that middle 2x4 is not centered, due to my amps being different sizes. Also I ripped the 2x4s down the long way for added clearance. To secure the amps down I cut some strips of 1/2" pine and bolted them down thru the cargo sub floor with sheet metal screws. Then screwed the amps down into the wood. They arent going anywhere.
The metal tab used to hold the front of the stock floor down was a perfect grounding point for my amps, so I notched the framing around it for clearance.
Finished floor. It looks stock to me, got lucky with the walmart carpet closely matching the interior carpet. And now I have more storage room under the floor for some tools and jumper cables. I also installed peel n stick membrane and sheets of lead down over the sub floor for soundproofing. Its super quiet back there now.
I bought another roll of the same carpet to cover my sub enclosure I built also so it matches.
Here's an added tip, I installed egg crate foam on the inside walls of my enclosure and it sounds alot better now, no distortion and hits slightly deeper now.