Just brought a toilet cistern Fluidmaster PRO45B bottom inlet valve.

It has to fit in an ancient Tyford ceramic tank on the WC which has
quite a large hole for the inlet valve to sit into and also a
complication is that the ceramic washer face is also at a bit of an angle.

Consequently tightening up really a lot on the back nut, it still
wobbles and leaks.
I read many times don't overtighten i.e. finger tight as much as you
can, then a quarter turn with adjustable pliers. But i have been going
at it way way more than that, with it still leaking.

As an amateur i can only think that a very big, very soft; sealing
washer to go on the inside of the ceramic tank is needed? Would this be
the way to go?

If so, is it possible to get such a sealing washer; or if i was to try
to cut one out, what kind of rubbery spongy material could i buy to cut
up? Thanks for help.