Been gone for awhile, but I have done a few seats and an air stapler will do the trick. also best to do the recovering on a warm sunny day to help stretching the material. The best jobs I have done were using the cover from Image Enterprise. Fit was good and it has lasted a long time.
Attaching the seat to the sled , I found the easiest way is to run a couple of self drilling screws with the hex heads through the tunnel and into the seat plastic along the outside edge of the tunnel. Use a long extension and a good screw driver gun. Next time you need to remove the seats, just break off the screws and use some new ones. It doesn't take much to hold the seat in place and rebuilding the corners with welded nuts takes time and will still be prone to rust as that is where the snow and ice ends up under the tunnel. 2 screws per side does the trick just fine.