I don't have MP money!In the past to hold the cork gasket in place I have used a very fine film of grease to keep it from walking. Be sure to clean both surfaces with a very fine brillo and spray brake cleaner on rag and wipe.
I am big fan of hating trans. flush service and not changing the filter that's bull crap. Adding new fluid into a trans. with a dirty filter NOT. I changed my wardens trans. fluid last week on her 2016 chevy ls malibu. I found out the filter is internal you have to take down pan and remove parts to get at the filter to change which is not recommended. This car doesn't even have a trans. dipstick ! Had to pull off drivers side tire to ck . fluid. So eggszacty 4 qts. of was drained and filled with 1 gal. of Castrol dextron VI full syn. Not happy but done, car had 63k miles on it. $33 total. Good luck @NYH1
PS I heard @Randall Ave does rd. service