Getting the seat cover on the foam base can be a bit tricky as you need to do the job with hog rings.
If you just want to stiffen up the bottom, what you can do (if it's similar to the S4 seats) is remove the seat base from the frame and remove the foam and upholstery as one piece from the base plate. There should be some big openings in the seat foam under your butt that you can stuff with additional dense foam. This avoids the need to remove the hog rings and pull the upholstery off the foam.
Bear in mind it's been a *long* time since I had the seats apart and I did it on a later Spider, so my memory may not be entirely correct or appropriate for an earlier model. Hopefully somebody else who's done the job more recently can chime in. But if you can do it this way it can save you a lot of work.
If you just want to stiffen up the bottom, what you can do (if it's similar to the S4 seats) is remove the seat base from the frame and remove the foam and upholstery as one piece from the base plate. There should be some big openings in the seat foam under your butt that you can stuff with additional dense foam. This avoids the need to remove the hog rings and pull the upholstery off the foam.
Bear in mind it's been a *long* time since I had the seats apart and I did it on a later Spider, so my memory may not be entirely correct or appropriate for an earlier model. Hopefully somebody else who's done the job more recently can chime in. But if you can do it this way it can save you a lot of work.