Thanks to a recommendation from a forum member I contacted euromotorsports about just buying the mounting plates from their Boxster rear speaker kit. A great guy Loi helped me out and they now offer the plates as a separate ebay item
1997 2011 Porsche Boxster Custom Rear Speaker Bracket Set | eBay
I tried modifying the existing mounting plates and even cut out new lexan plates but these are much better and look more professional.