Hi I'm from UK and love your seats and am planning similar.
I have found these but could also be covered in a tartan or color cloth but could give you and idea to make your own...
https://www.sportseats4u.co.uk/product-category/racing-seat-harness-roll-cages-brands/reverie-carbon-seats/reverie-seat-cushion-sets/
Double sided velcro stitched to covers would work well on alu??
Boydyrs