Yeah, @hernandez.stella brings up a good point. I started with some thin plastic ones from the hardware store, but they kept slipping out. Ended up using brass shims because they're stiff and you can tap them into place. Made a huge difference getting that door frame perfectly straight.
Tapping brass shims into place is such a smart fix for the slipping problem. You could probably use a small piece of that thin sheet metal from the hobby store the same way, cut into strips. It makes me wonder what other random hardware stuff we're overlooking that would work better than the "right" product. Like, would a stack of old washers work as a super solid shim in a really big gap?
Wait, brass shims? That's wild (and honestly kind of genius). I've only ever seen the cheap plastic ones or wood. I can't believe they're stiff enough to tap in without bending, that must be so much easier than fighting with flimsy plastic. It never even crossed my mind to look for metal ones at the store. You must have saved yourself a ton of frustration getting them to stay put while you were working.