I am replacing a previous owner's Bose metal exterior 'house' speakers he stuck along the sides, and prefer not to cut holes in the side panels of my classic Scarab. Are there molded plastic smooth-edge surface mount housings out there for like a 6" or 6X9 coax?

check Crutchfield and do search on eBay would probably be best starting point.

I have also seen some nice custom gel coated enclosures on ebay, but its been a while.