Whatever you're doing make sure it's sectional or telescopic so depending on the height at the venue you can change your structures height. The interior height may not be the factor either, depending on where your pit is you may be limited by venue lighting, ductwork or support beams.
If you're doing a single column for a sign for example, it would be best to attach the bottom to a piece of plywood that extends out from around the base. Just don't allow it to extend into the isles or adjacent pits.
Whatever you do just don't drill holes in the floor and install Redheads. Venues will fine you for that sort of thing!
