Choice of seats and suppliers, fitting etc.
The traditional low back seat is available from various suppliers like Intatrim (Discovery) and Interiors Seating (RX1191). A point worth bearing in mind is that the gap in the Sumo floorpan for the seat is relatively narrow so get a tape measure out to check what will fit and compare dimensions. Standard 2-piece seat runners lift the seat from the floor so bear that in mind.
Seat manufacturers offer various colours and this can be augmented with contrasting piping. Material used for the standard AC Cobra style seat cover can be vinyl or leather or a combination of both. Obviously full leather is the premium option but leather face / vinyl back is also very popular - after all, no one will be seeing or touching the seat backs as they fit quite closely to the rear cockpit face.
Because of the space available to fit the seats, production car seats aren't usually used as an option and happily most builders go for the standard bucket type seat - it looks right and suits the use of the harnesses usually fitted.
Seat buckets in raw GRP form are available un-upholstered for your own trimmer to do for you.
With the IVA regulations now requiring headrests to be fitted on seats at a pre-determined distance from the top of the seat back, this is an important consideration when specifying seats.
Having checked with the seat manufacturers, it is not possible for existing non-headrest seats to be reworked to add headrests due to the height of the backrest. In effect, the headrest will not be high enough and too far from the top of the (low) backrest to be IVA acceptable.
Some manufacturers such as Dax have made large shaped mouldings which can be retro-fitted to low back seats to give the required headrest. These are not so aesthetically pleasing but are accepted at IVA.