Winceed 9pcs Silicone Spill Stopper Set for Stanley Quencher H2.0 Flowstate Tumbler 40oz 30oz, Accessories for Stanley Cup with Straw Cover Cap, Square Spill Stopper, Round Leak Proof Stopper

this is something I didn't even know existed, and I've only had it for 2 days, and I already can't live without it. I bought this out of curiosity, and it works exactly as it's advertised, I can hold my 30oz and 40 oz Stanley upside down with it full of ice and water and it does not leak. I wish I had found these a long time ago. They are so good.

Worked great on my Stanley dupe. Only downfall is that it creates an air lock. No leaks.

Doesn't fit my 20oz (doesn't say to, but hey, a girl had to try!) and fits the older 40oz inside better than the newer one, but the gap filler for the straw section is a game changer on potential spills.

So I was a bit skeptical but these actually work for us.I love the clear cover as it’s not overly bulky and it works with my glass straws.

They fit snugly on the Stanley and easy to clean. I wanted to love them but because the cap is so tiny and thin, I feel like I end up touching my straw more which beats the purpose of covering it for sanitary reasons.