Let me guess - you're making a tray-table organizer, aren't you?

I had the same idea recently, and found the same answers that have been posted here. So I went with a very simple configuration to my "organizer" - I simply bought a cheap fanny pack from WalMart, which has an adjustable belt. I added a loop on the top of the bag so I can snap the belt around the tray table and adjust it for a tight fit, then throw the loop over the top of the table and hook it to the belt so the bag won't slide down. Easy on, easy off, and easy to carry when I'm boarding/deplaning.