Ok, I give up, I need to stop using my back pockets as treat bags 
I'm looking for something that doesn't need lots of opening and closing (I'll forget, bend over, and provide an unintentional jackpot for DDog) so would prefer one that's not a drawstring or similar.
I've found these Dicky Bag ones and when I saw them at a show they looked perfect, but at £18 it seemed a bit excessive www.dickybag.com/treat-bags.html
Has anyone got any recommendations?