How can I clean this off a bit better?

This is a Xiaomi phone, and they're very popular where I live so I'm trying to get some to fix. This old adhesive or glue, not sure which, is pretty annoying to get out even with a lot of IPA, and I don't have a heat gun or one of those heating station thingy, any other tips to go about it? I've already glued this one on top of this whole mess cause it was a matter of urgency for the friend that gave it to me, but I'd like to be able to do a better job in the future.