Since it is an Underground Tournament, you could very well do away with the ring all together and have the local be some sort of large basement or an abandoned industrial area. Or something similar. Or, if the ring is still needed, look for a local that has pillars that can be made into a make-shift, underground ring.
Example: In the movie Snatch, there is a scene where there is a bareknuckle fight between Gorgeous George and Mickey O'neil in a barn. http://www.hotflick.net/pictures/000SNC_Adam_Fogerty_004.html
You probably want to go a bit more industrial than this, but it's a good example of makeshift ring.
Just something that came to me. Best of Luck!