Your hinges may possibly be mounted a little bit off as well. Most freezer lids I have seen have slots for mounting them to make them adjustable. Close the lid, loosen all the screws on either the top or bottom of the hinge (the slotted ones), let the lid sit as flat as possible on the collar, then re-tighten the screws without moving the lid. This will help you get even pressure all the way around the gasket.