Two suggestions, neither of which should add much cost to your very thrifty tractor. First, you might consider putting a little roost, or maybe even two a couple of inches off the ground, across the back end of the tractor under the cover. In my experience, even your little meat birds like to roost at night. Another benefit is that it keeps them off the ground at night, out of their own poop, as well as allowing them to get up off of the wet grass if you have a big rainstorm.
The second suggestion is purchasing a nice tarp. I don't know what kind of shade your yard gets, but my little Freedom Rangers huddled in the shade for most of the day until I put a tarp over most of the tractor. You don't want to cover the whole thing, because it can really heat up if there isn't air flow, but a tarp can be adjusted if necessary to block the major sunshine while allowing for breezes. It is also nice if you have regular rainstorms, because it means your birds don't have to spend half the day huddling in one small section of the tractor, as well as keeping your feed dry.