Hi Mike, fantastic idea about a twin shot to achieve the seal! Have never thought of that.
Yes I know all about O-rings from my oil and gas days and the plan would be to use one of them for the tube entering the casing (you can see a rough groove location in the image), some sort of flat silicone seal for the main housing, then its just a tricky case (excuse the pun) of getting the o-ring and silicone gasket to marry up and crush well once the parts are screwed together… Thanks for your help!