LOL
Funny you should ask, that's been the subject of MANY discussions!
The thought was that it would "flop" so needed something at the back to support it into a circle shape.
But now that it's backed (with a very thin batting) it's actually quite stiff and self supporting, so I haven't needed to insert wire, or perspex, or thin timber, or anything, to make it hold its shape. I've used a small curtain ring on the back which hooks over a normal picture hook on the wall. It's sitting nicely with no floppiness.
I wonder if over time it will soften and start to sag, but will deal with that if it happens.