It was couple years ago, I honestly don't recall if there was a grommet or not. I'll be near the car this coming weekend and have a look - I'd go with the instructions and use the the tophat .
As far as the spillage, Seems odd that if the front was higher than the back the fluid would come out unless it was way overfilled. That typed, I never pulled the rubber plug prior to changing over to the camaro pan and adding a stick. I do know I overfilled mine by a pint and no leakage anywhere.