If it doesn't blow off in the first 48 hours of active fermenting, it probably isn't going to. You'll probably be fine here without one.
I generally start my beers with a large blowoff tube "just in case" and switch to an airlock after high krausen.
"I can't believe how many people think Air Lock is pronounced Hydrometer." -BigKahuna
"If you gave me a beer with placenta in it without telling me I would kick you in the nuts." -ODaniel
"We be in a big hurry for dope beer with much alcamahol and flavor, quality, balance, and aroma don't matter. We just wantz to be druck, u know?" -Yooper