This is a replacement boat plug that has an opening of about ¾" or about 19mm. It has a groove for grommet ¾" OD and 9/16" ID and 3/32" cross section.

Since you print it handle down you will get a nice flat seat and may not need the grommet.

Two varieties. My old plugs had a flat seat. I made a flat version and one that has a groove that fits over the flange at the entry. I don't know which is better. I would say the groove version has more surface making contact.

On my original plugs they ether did not come with an o-ring or they would dry out in the sun (where I left them) and fall off. It never seamed to make a difference in terms of seal.