Hullo fellow problem solvers. I had the same dilemma. The perished example I removed from my car about a decade ago I assumed was an original. It is a U section about 5x5 (probably the old imperial 3/16"x3/16") and, at the time, I found a near-perfect replica in Clark Rubber, so I purchased enough for my needs. The U section seats in the groove and provides a double lip seal against the edge of the rim. Very little water, if any, should get past.
Unfortunately, Clark Rubber here in Newcastle has reduced their range of rubber sections drastically and it is no longer stocked. It may be available at larger outlets. They also don’t stock the rubber buttons for the dipping reflector stop.
I have attached photos of the sections in question. Note that I have also included a view of some 1/8" vacuum tube which I think will be suitable. It is no more than 5mm diameter and has some ‘give’ due to the hole to compress a little.
(Excuse the fingernail, I didn’t get time to do my manicure this morning.)