I used Unistrut nuts and fender washers in the channel to bolt down the box. This was a bit of a pain because I had to epoxy the washers to the nuts to stop them from sliding around.
Filler plate to support box and carpet
New floor panel. This is clever because the same board becomes the bottom of the box after it's installed to allow for access to the under-floor storage. This way I don't have to carry the original floor piece with me.
Custom cut carpet. I traced the factory cargo mat onto some cheap rubber-backed carpet from home depot. This stuff is quite grippy (much more than the factory mat), but the rubber is so soft it rubs off easily, littering the floor with little rubber kernels.
Make some cuts in the carpet to allow it to drop into the underfloor storage compartment. Also made holes to line up with the unistrut nuts in the channel.
Place box over carpet and position holes over the channel nuts. Bolt it down using truss head bolts. This is pretty tricky and resulted in my cussing a lot and kicking a drill (drawing blood in my foot) but I eventually got it.
Floor board is still out, allowing access to underfloor storage.
Continued...