I don’t have one and I’m not losing sleep over it. You’re dumping it on dirt, not drinking it. The screen on top keeps the big stuff out and that’s been enough for mine.
I’ve started checking the remaining data right when I land somewhere new, since that’s the only time it actually matters. The expiry date itself is usually buried in the app and easy to miss, so I just look at the balance before relying on it. That’s about as far as I go with it.
Magnets aren’t the issue, the position is. Mine slid around loose in my bag for a bit and still blocked fine, but once it ended up folded inside a jacket pocket with a snap closure it kept working too. If it’s actually sitting against your cards you’re good.
The “no interference” bit is the only claim I actually believe. It’s a sticker, it’s not doing anything to your Wi-Fi because it’s not doing anything at all.