[elav]

There is a guy on eBay (search "Porsche Boxster 986 / 987 rear speaker bracket & grill. *Universal fitment") that sells a new insert to house a single 3.5" speaker per side. You could go this direction or seeing as you have the part, slide out the speaker holder you have, trace it on some plastic and make a version with a single 3.5" hole.