anything you can paint on can be a canvas right....of course it can.
Take for instance this washing powder box...
I made a loop from really stiff wire and poked the ends through the back of the box, twisting it and taping it down inside.
I glued the box lid shut and then covered the box with masking tape. Wrapped one way and then a second coat the opposite way.
making sure the wire loop at the back stays free.
Finally i added some thick gesso, leaving it deliberately rough. of course you could put it on smoothly if you prefer.
You can hang it up or have it free standing..
now just need to think of what to paint on it...any ideas???