Using the cardboard boxes in which I bring home my fruit and vegetables (and any other cardboard packaging I find) I construct a new flat box which is about an inch thick, secured with lots of masking tape and sometimes a few staples.
I then cover it with some fabric, usually something like a cotton canvas of medium weight which I have found in an Op shop. These pieces of fabric only cost me a few dollars. Once this is wrapped around the box I staple it on (sometimes making the corners neat is a bit tricky but not impossible).
Next I paint the canvas with very thin acrylic paint (actually a floor sealer) which soaks through the fabric and makes it stick to the cardboard. Then I apply a few coats of a standard acrylic ceiling white. It is quite amazing how much of this the fabric will absorb.
At this stage the canvases start to look like a good base for an artwork, though the white base coat also shows up any joins in the underlying cardboard or lumps where the fabric has not stuck very well. Certainly, some of my canvases are smoother than others but with my recent work I have discovered that once collage and paint is applied these irregularities seem to disappear, or even add to the effect. A final coat of a high quality acrylic varnish protects the painting. The final painting is solid and appealing because of the slight bumps!