Yes I removed the composite false floor thingy to get more space. Box sits in the hole that was left after removing that false floor. I used 18mm thick plywood that was doubled (glued together) on the sides and 4x near the seats. Probably way too much material and its quite heavy, but I have never done anything like this and carpenters I contacted were not interested. There are no screws, the box can be disassembled by just lifting the parts. I added some felt furniture pads between car plastics and the wood to make it even tighter fit (and to not scratch the car). Rear of the box was 4x 18mm thick since one plywood cover plate was using half as support and other plywood half. Cover and basically the box is also held in place relation to each other with long bolts that drop through drilled hole in the cover to the 2x sides and 4x plywood. They are there just by gravity and no tools required to install. These bolts or holes are not visible in the pictures since not done yet at that point. I used the idea from the video I posted in this thread.
Yes most definitely you could make a drawer. And actually could still access the space below the drawer but drawer would need to be pulled out and then open the hinged cover for the access. Just need to use more creativity depending on what you want. And if you have the skills or motivation/patience then go for it. I did not so I went with simple that works for me

Also I though that the cover could open by sliding or hidden gas strut hinges (or some kind of hinges) could also be used so the whole cover would rise flat and could serve as an table while access to storage is open.
Dimensions for the box are 20cm high. 4x 18mm part near the seats 82,3cm long (there is cut out in the center since the floor is not flat at least on GT). 1x 18mm part near the rear is also 82,3cm long. Side 2x 18mm parts are 81cm long. I also made center support for the box since my plan was to enter the car through the rear hatch, but actually we have never yet used it and have used rear doors instead. So the support is pretty much useless and overkill again, probably 15mm cover would have no problem even without the center support and 80kg on one point. But at least its on a hinge so I can turn it away if I need to put bigger stuff there. Some knowledge of plywood strength would have been useful

I have to see if the rear hatch will be used in the summer time when its actually warm. But beware this box like this weights many many kilos

Maybe I could still make round cut outs to get rid of excess weight without making it any weaker, but I'm too lazy and don't have the tools.
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The cover is 15mm thick plywood and again, maybe an overkill. Here are some dimensions in the picture. The 81cm part with the hinges goes couple of cm over the rear seat edge, so the 4 hinges support the last plywood part. This was my plan to see if it can be only supported in air only by the hinges and be careful not to put too much pressure there since its only our heads there. Then after install the last plywood part actually comes some 5-8cm over the middle console, so I just put foam support in between the console and plywood since there is a 3-4cm gap. That worked just fine and gives a lot of support, so now I only need to be careful with not putting too much weight on the corners, maybe it could take it but don't want to find out
Hinges are there because when you want to drive the car the seats need to be moved in the normal position and plywood needs to be folded so the seats can fit. Mattress needs to be lifted or rolled back to be able to turn the plywood. I was also thinking that maybe some sliding solution would be also an option, but this was easier for me since I'm not an expert in woodwork. That's why I wanted a professional for skill and ideas, but no one I contacted answered or were interested in this kind of project. Whole cover will fit in the trunk in case rear seats needs to be used, that's a second function of the hinges to get the part to fit in the trunk area.
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