Bath Overflow drain Gasket issue

I'm installing a new Americast 5'-0" tub with a Watts brass tub drain assembly.
everything was moving smoothly until I went to install the tapered washer on the backside of the tub overflow.

I understand that most of the time the thick side of the washer goes down. In this case, I cannot get a tight seal between the tub and the overflow flange with the supplied tapered washer. IF the bottom is tight, there is a gap at the top. It appears that the angle of the tub and the angle of the overflow flange is nearly identical. I cannot find a thick flat washer locally for this application.

Any suggestions would be greatly appreciated.

Turn the tapered washer sidewise. I install them that way all the time in that situation. All you're looking for is a tight seal all the way around. There are tubs that need the taper at the bottom, but certainly not all of them.

I have this same problem right now. Ideally I need a flat overflow washer. Wouldn't turning the beveled washer sideways put the overflow at an angle an then create problems when you go to install the bracket and overflow cap (screws would not go into the overflow threaded holes perpendicular to the tub, but slightly angled instead)?

I did find this clip on washer and was going to give it try, unless anyone can tell me otherwise that these don't work well.