Right now it’s just sitting there. I was thinking I would drill holes in the plywood where the cargo hooks go, remove the hooks, and drill a 3x3” or so square of 1/8” plate with the same hole pattern and use that for a washer on top of the plywood and bolt it down that way. I will need to sink the whole thing into the plywood for the drawer side mounts (or route slots for the bolt heads in the drawer bottoms), as I don’t plan to use any sliders in the drawers, just wood on wood. This will minimize cost, maximize space and make it very easy to remove the drawers (thinking removeable kitchen drawer with legs) or swap for a different drawer for different applications. Also the whole drawer side (or fridge side) can be removed separately as the need arises.
That's exactly what I am thinking for the tie downs. Then I'll route grooves in the top for seat track style tie downs. I agree on drawer slides. I am thinking I will sand them nice and smooth and add some teflon strips to make them slide a little better. I'm on the fence about a removable kitchen drawer. I want my finished height pretty low (wheel well height) and that doesn't leave much room. But as you pointed out, once you get the frame built, the drawers themselves are modular and you can play with it as needed.
I am willing to pay for some things, but I can get all of the tools I need and enough materials to mess it up a few times for what a set of pre-made drawers cost, and when I finally get it like I want it, I will still have the tools. And as you point out. I can make modular drawers or change my mind about the set-up.