When the old seal went I tried to get a new one but they aren't available so, in a panic situation after the clip on valve covers leaked worse than what I had, I got a length of moderately soft rubber hose that fit fairly snugly into the hole in the valve cover and the bolt fit nicely tight in side. I cut the length I needed out of it but cut it a bit long. I then put the washers and nuts on and tightened things up compressing the hose to the side of the valve cover and the bolt.
I did it along time ago and I think this is what I did. A last stage panic thought that so far (knock on wood) has worked.
Update: I forgot to mention that the two bolt holes were also counter bored. In other words a flat bottomed hole drilled part way through the valve cover. Do you want a picture? I will not take the fastener out though

Lee
Correction. I used the wrong term "spotfaced" but corrected it with counter bored. I remembered the symbol for it but not the correct term

"Spotface
A spotface or spot face is a machined feature in which a certain region of the work piece (a spot) is faced, providing a smooth, flat, accurately located surface."
If you had the head of a bolt riding the radius of a piece you could use either a radius style of washer or locally remove the radius for the bolt head.