Just build a plywood box that you can sit on sawhorses or something to keep it off the ground a bit. Then fabricate a frame and cover it with hardware cloth for the top.
But really, we need other info in order to really help you more. Are you in a cold climate? Are these going to be outside outside, or will they be outside in the garage, in the barn, in a shed, etc.
I don't think I would be brave enough to have them completely outside this time of year, at that young of age. I think you may run into heat issues with them. Even using a heat lamp, you are going to have trouble keeping them warm at night unless you build a completely sealed box for them, and I am not really comfortable with that. It starts to get to be a fire hazard and stuff then.
Let us know where your at, and more details, and we will see what we can come up with for ya.