How to call the outside of a hexagonal prism?

our reactor model has a hexagonal zirconium shell so i have to fill the region between two hexagonal prism with material ‘zirconium’,but according to the usr guide i can’t call it with ‘+hexagonal_prism1 & -hexagonal_prism2’. so what can make it works?

@flowersoul Use ~ operator

shell_region = ~hexagonal_prism1 & hexagonal_prism2

many thx !!!!!!!!!!!!!!!!!