Overflow gasket wrong taper for old tub

The old clawfoot I'm installing needs an 8 degree tapered gasket for the overflow to be vertical. All I have found so far are 11 degree gaskets. I've only seen the harder rubber ones; would the sponge rubber one deform enough for me to go vertical the extra three degrees? Any ideas where I can find a specially tapered gasket if the sponge one doesn't work?

I tried it with a flat washer and it works! Doh! The overflow fitting is already at a little bit of a taper, just the right amount.