You may want to visit your local hardware store and see if they have o-rings that are a similar diameter but thicker.
Another option is stretching the o-ring around the brass portion of the RM2 above the 510 connection. You may get more placement options with the o-ring there instead of inside the catch cup. As an added bonus, it'll hide the brass when attached.
Another option is stretching the o-ring around the brass portion of the RM2 above the 510 connection. You may get more placement options with the o-ring there instead of inside the catch cup. As an added bonus, it'll hide the brass when attached.