Sounds like the PVC pipe reducer to me - use a 3" to 2" PVC pipe reducer with "legs" to the side, a hose clamp to fasten a bag to the small side of the reducer.... Happened to me once when I covered the pot to speed up the start of the boil. I've since moved to a stainless hop cannister but still have Spider Mark 2. I don't know of a way around it, even open the PVC will eventually deform.