I installed a full width cabinet across a Scamp 13 and a Scamp 16 with our adding external screws.
I took some screws out of the rear window frame and mounted a piece of aluminum piece of right angle using the screws removed. This suppoerted the rear of the shell. I piece of 1x3 inch from the two side windows
to support the front of the shelf.
I cut a piece of 1/4 inch plywood to fit, sort of a half moon (the hardest part).
This piece of plywood sat on the rear aluminum and was screwed and glued to the 1x3. I made a front plate to fit between the plywood and roof. I cut three openings in the front to provide access and a location for doors.
More details can be found in this link.
We have the front couch/bunks and side bath. We never intended to use the bunks but recognized the space above the couch and between the front window and bath was wasted.
In this space we built a large 2 door cabinet for storage still allowing someone to lie on the couch. This cabinet was deep, the same as the bath and very useful for clothing storage.
As well we built two large drawers in the drivers side dinette and a large drawer under the front couch.
If you have questions don't hesitate to ask, I installed it 10 years ago and the details escape