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.