Stupidly specific question: how to secure a painting to a table or something like that

Hey guys,

So I've tried googling for this but I'm not really getting useful results. It's not really a painting as I said in my question, it's actually a whiteboard, but it's the same principle.

The whiteboard is 2 feet x 3 feet in size, and maybe 1 inch depth. I want to prop it on a table since I don't have a wall to use. I'm not really sure how to keep it up on a table or box like that.

I tried putting cardboard on the back of the whiteboard and the table surface to secure them but it does not feel sturdy at all. Should I put a larger piece of cardboard from like the middle/top of the whiteboard as a stand? Or is there something else I can do?

http://www.shrowling.com/upfiles/Stand-mirror-wall-mirror-SM-003-main-s-sm-003%20ya.jpg

Imagine that, except with the dimensions I stated above - how would you secure that on top of a table, upright, without it falling over?
Thanks for any help!