Take a look on ebay as well. Or maybe boats.com. Maybe there's a boat in Florida, Alabama or Louisiana someplace that was wrecked in one of the hurricanes and is being sold for parts or salvage. You never know! Don't lose heart, but at the same time consider a custom made one if all else fails...
If you have a nice Carver dealer in Ontario (the ones here in NY would likely tell you to go scratch) perhaps one of them can research some plans or diagrams or installation instructions or something from back when the Riviera was new. Its possible and worth a shot as well. The worst anyone can say is "no", right???