Unfortunately this can't be done, sadly, because it would be a great feature. I found these two Jira Issues:
The users basically said that the hours should be by project and not a global configuration. Altough, this wasn't implemented yet.
I would recommend you to create a Jira issue for this matter because it would be a new request.
Neither can be done in the Tempo Plugin. There are two issues but in the JTMPO project:
These two are exactly what you are looking for.
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