Thanks for the replies.

I got the Parker book and couldn't find the "style" that I want to use:
an O-ring in the bottom of a well, with a cylinder pressing on it. As:

||_______||
|O_______O|

I thought that I had seen such a use, but maybe not.

Not that it matters, cause I don't really have any flexibility as far as
the sizes go. So, I'm going to make it and see what happens. I'll let
you know.