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.
