You'll probably want to install at least a partial bulkhead as well to support the end of the berth platform, and this can be done by the normal tabbing method. You will need to grind down to fresh glass to get as good a bond as possible.
Also Epoxy resin is going to get you a better secondary bond than polyester, but that precludes using matt, you'll need to use cloth or roving.
Do some very careful measuring first... you say you have "about 6 feet" of empty space.... make sure that what you end up with will actually sleep a couple of adults. The further up the boat the berth is, the narrower the foot space and that makes for more conflicts in the middle of the night.
Add a more complete bulkhead and you may be able to add a hanging locker across from the head if you don't have something there already.
Good luck, keep a hatch open while you're working! It goes without saying that dust masks, paper coveralls etc are mandatory.
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)