Since I'd asked questions in this thread I thought I'd show what I ended up doing. My use case is a little different, in that I don't care about overlanding, drawer storage or sleeping inside, but I do want as much storage behind the second row as I can get. The Goose Gear platform would have worked OK, but it seems really overpriced, and still wastes a little space around the "wing" areas, and in what would be the footwell of the 3rd row.
The first step after removing the 3rd row (documented by others earlier in this thread) was sound deadening. Noico 80mil sound deadening mat went down first. (
Amazon.com: Noico 80 mil (2 mm) 36 sqft (3.4 sqm) car Sound deadening mat, Butyl Automotive Sound Deadener, Audio Noise Insulation and dampening : Automotive) This stuff has gotten stupid expensive, but I had a box left over from doing a similar job on my Excursion in 2019.
After that was down, I covered it with 1/4" Vibro closed cell foam (
Amazon.com: Vibro 228 mil -1/4" Sound Dampening Insulator-100% Waterproof Closed Cell Foam –Car Sound Deadening Material - Automotive Sound Deadener 9 Large Sheets-Buy & Support Made in USA- Not Russia or China : Automotive) Again, I had some left over from my Excursion project.
I used 3/4" oak plywood from HD, but if I had it to do over again I'd just use 1/2". I had some roughly 1.5"x4" hard foam blocks that came as packing material for something, and I cut them 1.125" thick to use as support for the plywood. That was as thin as I could go and keep the wood up off the smart key module and plastic trim mounts that are attached to the rear floor. If you didn't sound deaden, you'd need something closer to 1.5" thick. I was going to emulate what Youtuber Embark with Mark did on his 4Runner and build vertical pieces to support the stock plastic side wings, but decided I could make the floor flat back there just as easily. The main piece is bolted in back to the two outboard stock seat mount locations. The front of the board stops at the forward edge of the 3rd row mounting location, and is bolted to the outboard 3rd row mounts with some metal brackets I made. Goose Gear does something similar, but they continue their floor piece all the way to the second row to accommodate drawers. I didn't want that.
After that it was just a matter of covering it with carpet (
TrafficMaster Dayside - Color Ebony 6 ft. Indoor/Outdoor Texture Black Carpet MK22W180072S - The Home Depot), and cutting carpet pieces to make the sides look OK. I'd asked earlier about the rear trim piece, assuming I'd need to cut one up to make the back part of the floor look decent. That turned out not to be the case, though I'm still not wild about the unfinished corners. I ended up just cutting the carpet 1.125" long at the back, and tucking down behind the door seal.
It turns out the OEM rear mat, the one that covers the 3rd row when it's down, fits perfectly. Had I thought about it, I'd have mounted the OEM rear tie downs in a location that would have let me use the OEM mat turned around backwards, with its cutouts open to the rear tie-downs. As it is, I'll have to cut the rear mat to use mine.
I specifically wanted Captain's chairs, but the knock against them is they let stuff in the cargo area roll forward. To fix that, I cut a couple of identical pieces from the HD carpet, glued them together, then cut the resulting piece 11.5" up from the bottom. Some Velcro loop-side pieces hold them to the back seat, and let me use either piece individually, or both together.
This project added 3-4" to the depth of the cargo area where the 3rd row was, plus the area in the footwell. From a purely volume perspective, that's probably an additional 6cf. Considering that's the difference between the cargo area in the RX we had before and a stock GX, I've essentially doubled the difference between the two vehicles, and cargo space was the primary driver in the swap. The tale will be told when I actually pack the thing, but I'm quite pleased with how it turned out, so far.
Mark