We all have one of these laying about from bouts of highschool delinquency don't we? Even if you don't they sell similar metal signs all over the place.
Regardless - DO NOT TAKE ANY STREET SIGNAGE JUST FOR THIS CRAFT PROJECT!
That being said this was really quite easy to do. I just took 2x4s and screwed them together to form a box. Then I drilled holes in the bottom and attached casters. Then I attached the frame to the sign with construction adhesive.

I use it for a rolling footstool or to set my drinks or plants on.

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