Here's a pic I found on my phone...
First, from this thread, I steal this pic of what the bottoms look like on the 3rd row seat.
https://www.gm***************/thread...bracket.21499/
They have a plastic bottom, and brackets that clip onto mounting bars. So most people build a frame and then attach to the floor. I wanted space for my amps, and seat to be a little higher for my kids. So I stripped the bottoms and then started removing any cable mechanisms, brackets, etc, that I didn't need. You see the end result, you get to a seat frame with nice flat spots to mount on. They already have bolt holes and you can even reuse the bolts. Then you put back on whatever trim you need to keep things looking nice at the end. All the modifications are only visible from below, so no one can see them.
Again, these are slightly different than yours but should be similar.