I had a couple of minutes spare this morning so I cobbled together a rough idea for mounting an Elise seat into the 'F:-
You can see the original seat rails at the bottom, with the two seat mounts fastened accross the top. The black bits are nylon spacers.
I don't know how much you know about elise seats but the rear is side mount and the front base mount hense the funny configuration.
This is just a rough idea as I have no dimensions in front of me but it's drawn in 3mm thick steel which would be plenty strong enough.