Here are some possibilities.

• Mets-toi à l’œuvre, Walter !

• À l’œuvre, Walter ! — , Walter, à l’œuvre !

• Walter, mets-toi à la tâche !

No native speaker. Just my two cents:

Walter, au boulot ! Au boulot, Walter !

or even better (following user @jlliagre suggestion)

Walter, au travail ! Au travail, Walter !

(see the comments of users @livresque and @jlliagre).

As I mentioned in a comment and then neglected, you can simply say:

« Walter, au travail ! »

« Walter, au boulot ! »

Au travail is probably the more formal, but not by much. Both would work.

Thanks to @Dimitris and the community for the nudge.