It seems people haven't learned from the debacle with the dbrand Project Killswitch case using magnets that causes problems with certain Steam Deck fan speeds.
Recently, a company sent over a Power Bank to look over and potentially do an article and video on. After spending time with it and being ready to do a video, the company then sent a follow-up email to warn me that it's actually a bit problematic. The issue: the Power Bank uses a magnetic mount to attach it to the Steam Deck, and magnets do not play nicely with the Delta fans (which Valve still use in addition to the Huaying model).
So I made a decision not to cover it in a video (and article review) and instead do a PSA (public service announcement), because no doubt a few people won't check their fan. This is the item in question, and really: just go for basically any other Power Bank that won't potentially mess with your Steam Deck. Even on the Huaying fans, it could still potentially cause problems. In a lot of cases the Steam Deck already gets pretty high temperatures, adding to that is not going to be a good idea. Why risk it? Don't.
So what Power Bank to actually go for? As long as it has at least 45w delivery through PD 3.0 and a good battery size, you should be fine for actual charging. Just again — don't mess with magnets. The Deckmate system is a good way to attach things to the back of a Steam Deck.
To companies and people making things for Steam Deck: just stop using magnets. I get it, they're cheaper than designing a locking method but please stop.