On my upper cabinets I bought 12" vertical cabinets and cut them to fit.
The top and the back are fastened to the shell with glue and screws through the fiberglass.
I have added triangular blocks to reinforce the back where I glued a furring strip to the bottom
I reglued the insides and outsides of the cabinets for greater strength.
The oak cabinets serve to reinforce the shell when glued directly to the fiberglass and screwed in place as well.
The 12" deep cabinet fronts really stiffen the roof
Where I made bulkheads I used 1/2" exterior plywood and first fitted a cardboard sheet to match the curvature of the shell.
In all places where I added interior structure I cut away the ensolite and used a small wire wheel to clean the fiberglass and either epoxied in the bulkhead or bedded it in a polyurethane glue/sealant.
The bulkheads were also screwed in place through the fiberglass with the washers previously shown in my posts.
For the bulkhead across the front I epoxy fiber glassed strips in the junction on the hidden side.
For the bulkhead next to the door I used 3/4" exterior plywood since most of the panel was cut away for the cabinet door openings,
Basically I made patterns out of cardboard and then fitted, glued, and screwed.