Filling in the Strada seat wouldn't look so good in my opinion as well as ruining a practically irreplaceable set of plastics...
You can use the strada seat pad however with a glass fibre single seat unit. It involves cutting the seat unit in the right place to allow the seat pad to still contact the subframe. The seat unit then kind of fits around the seat. If you do it right then it looks as though it were made that way. Both AKA.Eric and myself did it that way with our Ricambi Weiss units (£55 ebay jobs
http://snipurl.com/10qtb ).
Darren