OK, parabola it is....

thanks to everyone for some excellent references and input.
For those that had questions:
I'm trying to turn a 'gravity well' as seen here:
https://www.spiralwishingwells.com/guide/physics.html. They claim to use
Y=-1/|x|, which would be a parabola, so I guess that's one more
confirmation.

I  glued up rounds 2 and 3 today (1.5" thick was the first one, these are
3" and 6"). The 6" should be plenty deep for a full parabola.
the plan is to have the outside edge be level (horizontal). There will be a
'wall' around the outside to hold the ball bearing until it slows enough to
start falling towards the center. The curve will progress to the center,
where the surface will be vertical. My guess is that friction and spin will
mean that I might end up doctoring the curve a bit, but then, the chances
of generate a perfect mathematical curve are pretty unlikely anyway, so it
will be a start...
I'm thinking I'll try both the parabola and the asymptote and see which
works better. And maybe an exponential one too.....
-James