I found an old broken adding machine at a flea market -- the machine didn't work (and was not worth fixing), but the plastic housing was in really good shape.
I began by removing all the inside parts (and saved a few of the parts for a future project).
Next, I made panels from some scrap black plastic to cover all the holes and glued them on the inside using a waterproof silicone adhesive.
The final step was fill it with potting soil, and select an appropriately sized plant. Since I planned to put this under a covered patio, I picked one that does well in the shade.
This was an easy project, and makes for an interesting planter!