This is the bare minimum gutted from the project I made that uses signing & verifying with Keplr mobile, due to their app being unfinished, I had to improvise, adapt, overcome.
Latest keplr update to V2, broke the message and it now says "Failed to decode message" so user doesn't see the message but apparently it still works, feel free to test it out & improve it.
Easy way to test is to modify the message after signing it, and see if it still verifies.
Forks and PRs are welcome!
Note: not all the dependencies are used, I just left them there from the main project, feel free to remove them.