I think there is a misunderstanding. I get that you want to leave the rafters protected with hardware cloth but not put a roof over it so that it's open for ventilation.
It's a lot easier to roll out hardware cloth and attach it than it is to cut each piece and attach it between the rafters.
On a project a few years back, I had this idea that I HAD to put hardware cloth between the posts. It made logical sense and was a pain and time consuming to do.
Then I had my "AHA!" moment and said, "Duh, I could wrap the cloth AROUND the outside instead of cutting pieces and trying to attach them to the posts." So much easier and faster.
If this won't work for you, cut the hardware cloth and staple it to the wood. Then hammer in the staples to make sure the wood staples are secure. Best to put another piece of wood over the edges of the hardware cloth so no varmint rips the cloth away.
If you are putting hardware cloth down the side walls, I would do it the same way.
If I am misunderstanding what you are asking, I am sorry.