yup. thin washer. one thinner than the oring, even when compressed. as the oring compresses, if it tries to increase in diameter, it is stopepd by the ID of the washer. so obviously washer and oring diameter have to be larger than the hole in the tank. optionally, a piece of silicone rubber sheeting could be slipped in there. :)
anything to make a flat face seal.
if you wanted to get fancy.. you could take your steel blank plate. mark an oring profile on it, and then dremil in a groove or use a broke wood spade bit that had no center point but did have side points.
I think the washer is the easies. you used to be able to get bonded washers at hyds places. esentially a thin washer with an oring 'glued to the id. oring was thicker than washer.worked ok on leaky banjo connections.