I carve the pumpkin, and it looks good for about two days before withering.
Lightly bake it after carving maybe? (what temperature and how many minutes?)
Is there a way I can soak it in some preservative like formaldehyde without starting the Great Fire of London?
Easiest things is to wash/disinfect the surface before you carve, like give the pumpkin a nice bleachwater bath. Use clean tools & seal the cut edges with petrolium jelly or WD-40.
Long-term you probably could dry it as a gourd to preserve it, but it’ll shrivel up a lot and look like shit.
Formaldehyde/pickling would also probably work, but i imagine it’d get all mushy.