trans pan leak battle

My last 904 pan gasket leaked all the time.....constant dripping from the edges. Reseal it 2 times, same result.
Now I installed a newly rebuilt 904. I figured that the tech installed the pan gasket, so all should be good. Installed it, and yup, it leaks around the edges of the pan :(

So now I need to remove the pan and regasket it. I've been using the rubberized cork, not the $0.99 cent rubber strip one. I remember there used to be a thicker o-ring type gasket, but I can't find a part number. anyone have it or advice on how to seal the pig? thanks