I come prepared with various sizes of ziplop bags,(esp quart and gallon.) and lots of them. Sometimes I'll bring small tupperware type things. I can't eat a lot at one time, so I really like packing things up for for later in the room, if it's something that can stand the heat and transportation. Then toss them in the backpack.
Sometimes I ask for an extra plate or the cardboard tray, to put on top of the food in the ziploc bag, sometimes can get that.