Richo, just curious, 240v and 30-50 amps for your equipment? What kind of equipment uses that?
There isn't any equipment that uses 240V. It's a "portable" sub panel with multiple 120V circuits as most venues don't have 4 circuits along one wall or in one corner.
Buy or make cheater cords (with some limitations). Here you want an adapter with a 4 prong receptacle for your portable subpanel and a 3 prong plug (hot hot neutral) to connect with the venue's electrical system. Have a green pigtal coming from your adapter to connect to a known ground.
The only easily accessible known ground in many places is going to be at the 120V receptacles and that is usually going to be only 14 gauge wire. Is that sufficient ground for a 50A panel?