I use a couple of Mr.Beer LBKs aswell as some 6 gallon plastic buckets. In fact, both of the AG batches I've done recently have been in the MrB. LBKs. A couple of batches ago, an extract brew, I had the LBK filled really high (like maybe 2.8 or 2.9 gallons, assuming the actual volume of the LBK is about 3.15), had some amazingly active krausen action going and at peak it was exiting from the lid and dribbling down the sides. Not a huge volume exiting but pretty consistent for a day or two. Wasn't a problem and just having the LBK sitting in the swamp cooler was all the precaution necessary.