Based on the cramped quarters, I had to bring my PVC overflow drain up
parallel to the back of the tub (rather than directly vertical, at an angle
to the tub). The very think neoprene gasket that comes with the kit is
angled (one side is thinner than the other to make up for the slope of the
tub). Since my PVC is parellel, no matter which way I turn the gasket, I've
got a loose edge that would leak if water came through the overflow.

Do they sell flat gaskets for this purpose? If not, any recommendations on
alternative materials I could use?

Thanks,
Keith