jfaulken,
Have not seen them for sale. But, you are on the right track. We built ours years ago.
It is a 8'x8' cyclone fence design cage with posts at all four corners and a rail at top and bottom on a 3 point hitch set-up. Braced from post to post (lower to top) with a trap type side door.
We back up to the calf or near by and push the calf inside. Gotta pick the time to grab the calf. Once inside we are relatively safe. If need be we strap the calf to the inside-side with a strap around the neck just behind the head and straps behind the front legs and in front of the back legs and head to the barn. Mom can see the calf and will follow. If not,when we finish we just raise the unit and drive off leaving the calf there for mom's reunion. We try to get the calves born in the field within 12 hours. They are not so coordianted yet. The sides have been stretched a bit from a more agressive mom. It's a lot safer. If you build one don't forget to build in a small wire type shelf arrangement for box of instruments,meds,etc.
I won't let anyone approach a new born calf in the open field. To many near misses. We are glad to see an agressive mom so that we have a chance to out-do the yotes,etc.
It works.