I'm assuming you don't want to alter the looks of the exterior so... This is what I suggest. Get some Aircraft Grade 1/8 inch thick sheets of plywood. cut them so that they are the length and width of the bulge in the roof of the
Scamp. You may have to cut the sheets lengthwise into several strips to account for the slight compound curve in the bulge. Now, remove all the insulation right to the bare
fiberglass. Take the sheets of plywood, cover them in contact cement. And the roof too. Attach. Now, for extra strength you can cut every second layer of plywood with one extra strip so you don't overlap any seams between one layer and the next. (I would use at least 4 layers) After all the layers are in place, (And your helpers' arms are dead.) you can hold the plywood to the roof by spending $20 on a couple dozen pieces of 1x2 and cut them to the same lentgh as the distances between the floor and the new ceiling, then jam them in place, holding the ceiling up while it dries. You can also do the exact same thing (sort of) to the outside of the
Scamp. It would be even stronger and you could easily hide the fact that the extra material is there by shaping and sanding and
painting the plywood to match the original bulge. Good luck!