If I remember correctly, that Semcycle seat base is the same (or similar to) the Viscount seat base. I know people have successfully converted Viscounts to air. Try searching the forum for "Viscount air" and see what falls out.
My original plan was to glue the vinyl down over the sock. My bro covered the tube as is and now he is holding the sides of the cover down with a strip of duct tape running from one side to the other underneath the seat base. This was a huge experiment for me and it kinda got down two ways, so if this is confusing or you don't understand something, just ask and I'll be happy to help.
Good luck! *
And the last sentence should say "and it kinda got -done- two ways, so if...."