It looks a lot like a hutch my dad built for me when I was 12 and had just bought New Zealand Whites. We made it a mirror-image with a hardware cloth divider for, what turned out to be a pair.
A N Y W A Y ... You probably want to enclose 1/3 to 1/2 of it in plywood. You could insulate it, or not. We NEVER did and the bunnies were fine with straw. You would put the enclosure(s) where a door is, for easy access.
They got plenty of ventilation from the bottom and from the front.
Before any BYCer's hit me, I know that birds respire their "sweat", but so do bunnies. You might want to read the info pages (BYC) on ventilation before you make a decision, BUT, it's summer here, already, so you could take a month to decide, and the building project would be a quick one.
as much as I like the idea of being open, I do think I wanna have something in case of bad weather...I am not sure how well they would do in this set up without some kinda enclosure.
it will probably take me a good while to figure out. and then I'll change it. ha
I have a 2x4's structure that held a large barrel off the ground, thinking I might enclose that and connect it to hutch.
funny how last week this free hutch was all I needed. it just leads to more planning and designing and trying to figure out a new set up.
if anyone is like me when it comes to decisions, you're better off getting something pre-made complete for chicken set up. hahah
It reminds me of my first coop. ( 8 years or so ago) it too was a rabbit cage I got from my ex father in law. He liked building things and was good at it, he built it for his daughter who had a rabbit but she never took care of it and so they sold it and gave me the coop and a 8x8 run type thing that I used for baby chicks. The run didnt have a top, I had to put one of those on it. However the rabbit hutch he gave me had like a little dog house on one end. I put a light inside it and kept chicks in it as well but ended up changing that over for silkies. I dont have it any more but it worked great. I never had to worry about anything getting my birds. When some neighbor's dogs attacked my flock one morning the only ones left unharmed was my silkies that were inside that coop.