This will sound silly, but I just brewed up an amber and didn't have a lot of primary space, so I have 5 gallons in a 6 gallon BB with air lock, 5 in a bucket with lid and airlock, and about 2.5 gallons in a 5 gallon bucket with just a boil kettle lid on it. (My large fermenters are tied up with lagers; I thought I had one free, but didn't actually check before starting this brew. So, I have my first truly open fermentation going on and will bottle as soon as terminal gravity is reached. I think you and I will both be fine. Good luck!