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What are the best customer carry trays for concessions and where do I get them?

You'd get them from your regular concessions vendor.
Sysco sells small cardboard boxes and cup carriers (the kind In-n-Out uses).
VSA sells Chinet T-Slot food carriers. They're much larger, featuring 2 drink notches and a big open spot for nachos, hot dogs, whatever else you'd want to put on it.
Depends on what you sell most.
We wouldn't be able to use it because we sell way more sodas than we should.

There is a good supplier that is able to attach portable trays to cupholders which is worthwile looking into.
The email address is www.caddyproducts.com

We use ordinary little baskets from the hardware store. they cost about AU$2.50 each We write in the bottom "please return to candybar" and put contact over the writing. We do lose a few, but generally people do return them, we clean them and reuse. Its probably not feasible in a large cinema, but for small fry like ourselves, its great. We looked into the cardboard ones like McDonalds use, but couldn't justify the prices.