At first when I was getting ready to cut the supports, I just visualized what needed to stay and what didn't. I wanted to use as much of the existing hardware as possible and not take away the structural support. It was a little nerve wracking to be cutting things out of a new vehicle lol. Once I was sure, I used chalk to mark my lines and used a sawzall with a metal cutting blade. Once all the metal was cut, I put it back in place and put cardboard over top and carefully marked and cut it to shape. Folded it half and traced the other side which gave me a perfect match on both sides. I put the cardboard back on top, poked holes for where I wanted the bolts to go through and hand tightened them down to make sure it was a perfect template. Took it all apart and traced it onto 3/4 inch ply. I used a circular saw and jigsaw with the capability of angling the blade on a 45 to cut it out. I wanted to take the edge off the ply to make it a smoother transition from foam to wood once I placed the mat back down over top. Cutting the foam from under the mat was a pain, I used some black craft paint on the wood and pressed the mat with foam down on top. That gave me a rough idea of where to cut the foam. Slowly but surely, I kept taking bits of foam away until it fit smoothly over top. Measure twice, cut once.
I can say now, that it was so worth the extra effort and money. I went on an extended roadtrip this summer and I was able to hide all kinds of stuff in there. It's amazing how much space is wasted in that area. I should mention that there is even more space to be had under the front seats. There's styrofoam inserts under there about 3 inches thick (once you pull the extension floor out, you can see what I mean). I decided to leave them in there for two reasons. One, insulation and road noise. Two, if you pull them out, the mats won't sit down in that space correctly without cutting them, so I let it be.