I used the same Sparco-brand seat bracket with Momo sliders and a Momo seat (i.e. dissimilar brands). Just had to drill some holes in the seat bracket to mount the sliders.
If the Jeg's seat is too narrow for the bracket base, you could easily use some 2"x1/4" steel flat bar (found at most any hardware store), drill holes in the flat bar, mount that to the seat then drill/mount the flat bar to the Sparco seat bracket. Can be done with basic handtools (drill and a hacksaw).
It's MUCH easier to use pre-made seat bracket that is the correct height/width for your floor, as well as use the same bolt attachment points (i.e. bolt-in), THEN adapt the seat. I doubt seriously that you're going to find E36 seat bracket that's made for a Jeg's seat.
Regardless what you choose, please use grade 8 hardware.