The boxing gym upstairs at Marquette Recreation Center is not a fancy place. The thermostat malfunctions, the windows don't open wide enough to let in the breeze, and the punching bags are "borrowed" by pugilist wannabes who should know better. But dozens of fighters call the gym and its undersize ring home, some for a day until their fancy passes, some for years until they win the undisputed championship of the world. Cory Spinks jumps rope there, as does former International Boxing Federation champ William Guthrie. But even 90-pound weaklings can practice under the tutelage of trainer Kevin Cunningham and his assistant, Joseph Dunlap, so long as they sign a consent form. The cost is $30 per year (covers insurance); hours are weekdays 2 to 5 p.m. Staying out of Spinks' way while he's on the heavy bag is recommended.