I have seen 2 basic designs that look reasonable. One would use wheels. these tend to be bigger and heavier. usually made of wood.

the other is simple and light weight made of pvc tubes like a small hoop house and you drag it around.

There are tons of designs on the internet all for free.

the one I like is similar to this one but uses pvc pipe around the bottom edge instead of wood. much lighter. But of course an animal could get inside by going under the bottom.

Here is a link that might be useful: tractor