It's called the vaginal fornix. And - The Flex Company

Softdisc Menstrual Discs - 14ct : Target

I tried the Flex menstrual disc for mess-free period sex

Vaginal fornix - Location, Types, Function and Pictures

Creating period products for every body, The Fornix

Flex Disc Review: Meet My New Favorite Period Sex Essential

The Fornix by Flex®

Flex Reusable Disc, Reusable Menstrual Disc, Tampon, Pad, and Cup Alternative, Capacity of 6 Super Tampons, Lasts for Years

First Impressions: Flex Menstrual Disc Subscription

Creating period products for every body, The Fornix

Goodbye Tampons: A Period Diary Using Alternative Feminine Products — The Moné Edit