There are two 10mm bolts that cover this secret area. I made some toggle screws, so I can access the area easily. It is good for stuff you don't need very often.
Thread on the topic.
https://www.4runner6g.com/forum/threads/need-aftermarket-storage-solutions-for-cargo-area.3319/page-2#post-100337
Mods:
Winter Tires - Still waiting for snow to try them out. :(
FixcamX dash cam - Very impressed and discrete.
Discovering extra storage under the rear floor board of the non-hybrid.
Westcott Lo-pro full rack - Mixed... I love the low pro look on the sides and back, but i am not liking the...
I am very happy with the 2.4L turbo. Very impressive engine, and I don't even have the hybrid. I have met a few of the haters out there and most of them are usually the stick the head in the sand type of people.
Not a lot of space. 2 1/2 inches.
If you replaced the board with plywood/MDF and had a support in the middle, you could get another inch. The board is a hard foam that is quite thick. I didn't measure, just going by looks.
Here are the stl files.
I have included two files for the bracket. One with a simple hole and the other with a slot. The screw is okay if you print it horizontally. You may have to chase the threads with a die depending on your printer setup.
If you don't trust the screw, source one on amazon...
Here is what I did.
I replaced the tie down with a 3D printed insert. I then replaced the 3 clips on the board with some Velcro. It is nice to access this space. Thanks for creating this thread. I can keep a small socket set, pull strap, and jumper cables in there.