I agree with @ike, that's the best way, in my opinion, to go but as has been said I suppose it depends on where in the world you are and what you have at your disposal.
I've used ¼" ply before now for 'portrait' style work and it's turned out fine. These 3 were done with ¼" I too use a cheap ply painted black for a backer. Or you can make a frame. Here are the three types of 'frame' I use. 1) with the 'backer' bigger than cutting, 2) Frame incorporated into cutting 3) Glued onto a large backer and then framed with another wood.