You do need to think a bit carefully about it.
Most of our seats, such as the B6, are with a backframe so sufficiently self supporting to have threaded inserts in the sides or bottom for runners or side brackets. The advantage of this is that the seat can be moved back and forth if on runners and fittment is straightforward. But the seats are heavier and more costly.
Because the B3.5 does not have these, you cannot just drop the seat in and sit in it or it would flex and there is a risk it would crack.
Normally customers would fit a support to the top back of the seat and and a couple of supports to the lower sides and front lip. These can be alloy bar that is bolted to the seat one end (cup washers supplied) and to the chassis at the other end.
Or you can get a bin liner, put in the car, hold the seat in position and fill the bin liner with expanding foam. Take the seat out and trim the foam to clean up. You then have a strong and even support for the seat.
The most important is that the seat is supported.
These seats won't be for everyone.
If you need the seat on runners they would probably not be suitable without you spending time making your own backframe type support.
However if you want the seat fixed, want to be very light, want a top carbon finish and are prepared to spend a bit of time carefully fitting, then they represent extraordinary value for money.