You can take a piece of muslin, cut out a square several inches larger than the seat. Take a crayon or a piece of colored chalk, trace around the edge of the seat.
Then measure at the corners down to bottom of the seat. This will be the "side or boxing as we call it in the Upholstery business). Sew the "boxing" onto the seat. Make an elastic casing for the bottom edge (add an extra inch for the casing) and make ties with velcro in the back to wrap around the back (like apron straps).
This is so the cover can come off for cleaning if necessary.
It's basically a slip cover for the seat. It shouldn't bunch up under you or the passenger with the sides added.
Make a prototype cover first so you can make any adjustments.
Last edited by Chasing Hawk; 05-23-2017 at 01:37 PM.