Not really a knitting or crochet accessory but I just had to try these out. 

From Fox & Pine, here is what they say:

Accessorize your Stanley water bottle with our cute straw toppers. This is an exclusive Fox & Pine Stitches straw topper. The listing is for one item.

Made of BPA free food grade silicone.

Fits 20, 30 and 40 oz Stanley Quencher and Simple Modern Trek.

Ideal on 9mm straws but can also fit 10mm with more effort to take them on and off.

May fit other water bottle brands, please check diameter of your straw for an accurate fit.

I tried in on my Hydro Flask, both 20 oz and 40 oz and it fits very well indeed! :-)