I've tried a few methods and still came up wanting, till now. My
problem john had a flange that stuck a little too high above the floor
and was less than level as well. Aside from being annoying it ate wax
rings quickly.
My solution; I patterned the base of the toilet, and cut a piece of
smoked plexiglass roughly 1/2" bigger than the base (still a rectangle,
I didn't attempt to contour it precisely), rounded the corners and cut
a 7" hole for the flange in the appropriate spot. It cracked on the one
side of the big hole, but as I was siliconing it down, I didn't let
that bother me.
That 1/8" made all the difference, it is rock solid, and even seems to
work better, and the smoked plexiglass actually looks pretty good,
unusual perhaps, but good.