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Laverda explained this very well, you need the combination of firm and comfortable. You will also need to drill breather holes in the seat pan. P&K get there seat form from Bob, not very comfortable foam
A few at about one inch dia. I would not try to rivet the cover on, it will rip after time. I rivet these to a seat pan and it is easy to firmly hold a cover >>> https://www.upholsteryshop.co.uk/shop/upholstery-sundries/ply-grip/tack-strip-tacktite-30-1-strip/