The dipstick tube has a ridge/shoulder about 1" up from the bottom and is a press fit into the hole. Goes in dry...no o-ring/grommet, but when I replaced mine I added some sealant just for good measure. Then I tapped on the ridge with a long drift...carefully, just to make sure it was in tight. Been that way for a few years now with no leaks. My old one was cracked just below that shoulder and had holes where the upper bracket used to be.
Maybe your old one was broken off and you're fighting that piece?