That is a good point on no insulation in the lid... might need to get some spray foam and fill that thing out. My lid is cut for a pin lock but I have one true ball lock now that has a good bit of wiggle room. Still was fine all day in the sun but there is always room for improvement.
I used a roto zip with the circle attachment but I don't recommend it at all. That drill was like a hot knife through butter and any snag made it jerk and just disentigrated the plastic leaving very uneven cuts.
Yeah - I guess that is the way I will go as well, since you can't epoxy anything to the polypropolene effectively. I may look around HD to see if there is a decent rubber gasket or anything that might fit in the space as well. I'll let you know!
Maiden voyage with 8 Point IPA from Devil's Backbone in VA (hopefully I don't get banned for a non-home brew). Works like a charm. I used pieces from a flexible cutting board and white duct tape to seal off the foam.