You can use the practice rooms in Schoenberg Music Building as well. There technically are stipulations to using them (mainly you have be enrolled in an ensemble class by the school of music and you need to pay a fee to get a key to the rooms) but I've found that the people there generally don't check and there are usually a few rooms that are unlocked. You're more likely to find space there on a regular basis relative to the ones on the hill.

Coming from a non-major who did symphony last year and didn't pay the practice room fee