Take the floor out if possible, although if you are not going to try to grow things in *real* cold weather you could regard it as a shelf.
Put (preferably dark-colored) jugs full o' water in as much of it as you can manage, anywhere not actually occupied by plants. You can even set plants on top of rows of full water jugs. The idea is to maximize thermal mass so it doesn't get as hot during the day nor as cold during the night.
I'm not certain where you live or what you want to grow at what time of year, but if cold will be an issue, you might consider covering (or replacing) the north-facing wall of the tractor with one or more thicknesses of foam insulation, with plywood or something on the outside to prevent it crumbling in the UV and with a white or aluminized layer glued onto the inside to increase reflected light.
With a small greenhouse like that, you will want VERY adjustable vents - more than for chickens - and will have to learn to check them frequently during the day, especially til you learn how the thing behaves. It's really important not to inadvertantly cook the plants on a sunny day, nor to leave a vent open on a cold night
Sounds like a good project, have fun,
Pat
