I use a broccoli rubber band as a minimalist iPhone 'case'. [1] It makes the back and bottom of the phone grippy, so it's easier to hold. It does slightly block the corners of the screen, but not much. It doesn't provide much protection in case of a drop, obviously. But the added grippiness makes drops much less likely!

I have occasionally bought broccoli that I didn't need just so I could get another rubber band, after my previous one broke. They typically last 2 months or so.

1: https://imgur.com/a/E2yO6XX