This is what I have just done with mine:
I ordered some 125 x 125 x 3mm Steel plate of eBay, and cut out the middle so that the solid mount would fit inside. I also had some 3 x 30mm strip that was welded to the bottom edge to bolt to the floor. This was the mounts tacked together for a trial fit on the car:
![Sub-Mounts-3.jpg](https://i.ibb.co/gjJYzWW/Sub-Mounts-3.jpg)
I then drilled some 8mm holes in the plate and then offered this up to drill through the floor:
![Sub-Mounts-12.jpg](https://i.ibb.co/pK8Hn9G/Sub-Mounts-12.jpg)
When they were all painted and ready to assemble, the wax was removed from the toe board and everything was degreased and dried. I then used Tiger Seal between the mount and the Toe Board, and inside the car, I used some M8 x 50mm penny washers to help spread the load. These again had Tiger Seal on them to help keep the water out!