Hi Mark, glad your on the mend, it's hard to not go into the shed even when we're damaged . Years ago I pulled a tendon in my arm from repetition factory work but during the work cover time off I still went in the shed to clad the ceiling !!
If you do a bonnet to just skin it, it will become bigger, like wearing more jumpers, each layer makes it a few mm bigger. If the part your wanting is say , a modern stone guard inner arch cover, it doesn't matter as the size wouldn't be seen or affect the part and fitting.
Making a mold removes the outside size problems but can effect the inner if made to thick .
I'll never do a RS nosecone lol, I used to do them at Velo/quickco and hated them, just takes so long and frustrating... Gladly hand it to someone else to do.
Mk2 C pillar covers are different between 2dr and 4dr, as the 4dr ones sweep down around the door open shape. 2dr are basically straight . Also parcel shelf's are a different width, a 2dr one can be trimmed to fit 4dr but not the other way around .