Sandbox for testing artifact publishing process

Hi, I'm working on some gradle build infrastructure to sign and publish several artifacts. This is consulting work so I don't have access to the signing keys and sonatype credentials; I'm just writing the tools for my client to use. However unfortunately my script isn't working for my client, and all we have to go on is the dreaded "401" error. We've triple-checked that their sonatype credentials are correct and tried several other things, to no avail.

I was wondering if it's possible for me to set up a "sandbox" project in my own account using my own signing key, to allow me to try and iterate on my build scripts without needing to waste my client's time. What would you recommend?