Isaac,
I inherited the exact same thing on my 13
Scamp and just finished replacing it.
The PO had basically removed the window and glued the old AC in.
I used the same wood as my cabinet doors on the inside to trim the new AC out and on the outside used 1 inch wide aluminum strips screwed into the wood on the inside to frame the
fiberglass opening and strengthen the shell. I used PolyButyl tape under the aluminum strips. I also sandwiched aluminum roof flashing which comes in a roll, under the 1 inch and fills the gap to the new Samsung AC unit. The outside is now totally an aluminum appearance and the interior is cherry stained maple ply. I'll try and get some pictures up, but I'm out of town at the moment.
I don't think a window AC will ever be a work of art, but this installation now looks OK compared to the mess that was there.