Custom Seat Brackets?

A bunch of us have, and as an example these are Ricaro Sports out of a BMW 3 series recovered in two tone specifically for this car.

The brackets we made out of 1x1 structural steel box and mounted the sliders of the BMW seat on top of it.
The BMW seats are made for a flat floor, so the brakets took a little work. :-)

I should add, good luck using your original holes if you do anything other than what was in it.
You can however, use short "stove bolts" from the inside with a nylock nut underneath to plug the original holes.