Wasn't sure about the 4.2 cm inner diameter at first, but it stretches just enough to stay put without pinching. The black silicone feels sturdy rather than flimsy, and cleanup afterward took about a minute.
Thomas★★★★★
The silicone has just the right amount of stretch, and the 4.2 cm inner diameter fits snugly without pinching. I've used it a few times now and it stays comfortable the whole time.
Owen S.★★★★☆
Wasn't sure about the sizing at first, but the silicone stretches enough to stay comfortable during use. It does what it's supposed to, though cleaning it properly afterward takes a bit more effort than I expected.