hi dhamps,
my 2 cents....
You can create a volume in Pro/mech under
Structure --> Feature --> VolumeRegion-->Create
and once u've created the vol. try constraining it in all 6 degrees of freedom. I havent tried it myself yet but see if that works. Also if there is say a "cylinder" you wanna constrain then i would just constrain the two opposite faces to make things simple. hope that helps.
ym