Are you going to make one or repurpose something? I've seen people use kiddie pools, which might be good for that many chicks. You'll probably need more space than you think you do at first... they grow fast!
If you are building one, use a 4x8 sheet of plywood for the base and another 4x8 sheet for the sides. If you use OSB, @ 8 a sheet, you will only have $16 in the box. Use screws and you can disassemble and repurpose or to make easier storage for next time.
I just use my utility trailer, which a bit larger, 8x5. That is a great size for 18 chicks. They need lots and lots of room.
Nice utility trailer! Yep, I'd make one of plywood or OSB if I were going to do it again. We're using a big rubbermaid for 8, and they'll be outgrowing it any day now. It wouldn't even cost me anything extra (compared to the rubbermaid) to use a couple of sheets of plywood.