Whats under the front living room slide?

[ronenz]

I am planning to mount a custom built diamond plate 53" x 17" x 5" locking cabinet under my sofa where the pull out bed used to be. This is the slide with the one sofa visible in my profile picture. The sofa frame sits on what appears to be a box deck that is about 3 inches offset to the actual floor. The sofa frame is secured to this box deck with two screws that are not fully screwed in which make me wonder if there is a reason for that? I would like to secure my custom cabinet to the box deck with some 3/4 lags. Anyone know whats under the box or the dimension of the plywood (thickness)?

Thanks

[ronenz]

This is what I found out. What I though was a built up interior floor deck under the sofa is actually the exterior deck. Instead of lags, I was able to use carriage bolts with the nut end secured in the box.