To get started on a CompuCanvas, you will need a few parts that are probably most easily found at an art supply store. In my area, we have Michaels, which is where I bought the parts shown here. I wasn't able to find URLs for the exact parts I bought, so I will show and describe them here. I think you will be able to find similar parts at an art supply store in your area or online.
Perhaps the most essential part for a CompuCanvas is the canvas! For this project, I recommend an 11x14 inch "Heavy Duty" style art canvas. You could probably make it work with a 9x12 inch canvas, or go bigger with something like a 12x16 inch canvas and have more room to spread things out. (PyRulers for scale.)
Many of the art canvases I've seen are very shallow in the back - maybe half an inch - which is not a lot of room to work with. I have found some canvases labeled "Heavy Duty", which seems to designate a deeper design with about 1.5 inches of room to work with behind the canvas. I like to place USB speakers behind the canvas (so you hear, but don't see them), and the standard, shallow canvases would not have enough room in back for any USB speaker I've found.
Besides the canvas, there are two other essential parts for building this type of enclosure. The hanging wire and "D-ring hangers" shown here will allow for hanging the CompuCanvas assembly on a hook or nail in the wall.
The above parts are all you really need for a basic CompuCanvas enclosure. But for a more deluxe build, try to find foam board, like the 8x10 inch pieces shown here, and these 0.25 inch "offset clips". The offset clips will be used to hold the foam board in place behind the canvas. When designing a look for the canvas and laying out the electronic parts and cables, you may find that it works better to attach some parts to the foam board, instead of the canvas.