Update: Car is back together with the JB Weld sealer and so far, so good. I sprayed more B12 all over that area and no air leaks. I was a little concerned about the plastic also, but the seam has lips on it and the screws hold it down tight so it has two methods to seal. I didn't think to check the pop-off valve but at least, that's easy to get to.
Thanks for all the suggestions...... Bob