How rigid are these? Are they just mounted at the floor or do they have to be mounted in other places to give enough support?

I've got the high back seats from mk ( earlier ones were about 150mm shorter)they're quite sturdy ( 5-6mm thick) and can be bolted direct to floor or subframe.
they're a snug fit as long your ass is'nt too big!

as long as your ass isnt too big?? damn, my Mrs wont be coming out for drives then! will have to trade her in for a younger thinner model!

Thats why my missus wont be getting in my car, she'll only slow it down with extra weight!!! no,no,no.

I wont let let mine in the car any way. Too much washing up to be done.....