I don't know if I'd want an air tight seal there. Any moisture present will cause it rot out from the inside out. A/C oil is also like brake fluid in the sense that it readily absorbs moisture so now that it is open it is surely moisture saturated. You may want to make some plastic caps or something and drill a vent hole in it. It'll still run the risk of rotting, but hopefully the moisture will eventually evaporate.
BTW, that engine bay is immaculate. I'm jealous!