You can put a tarp on the barn floor and your shavings on that. Then a circle of cardboard, tall enough that they won't fly out at first.
When they start getting out take the cardboard away. Presto! Be sure to check temperatures under the light to make sure they can stay warm enough. Make sure no drafts exist (keep the cardboard circle small enough to keep drafts away).
I like brooding in chick n hutches, with plywood or plexiglass added on the sides to make them draftproof. I put the metal catch tray on top of the wire floor to use shavings at first. The only thing is they aren't terribly durable if left in the elements, and you have to loosen the screws to get the tray to fit inside. They won't fit 50 chicks, either.