Hiya! Although I have never actually built a rabbit tractor myself, I was planning on awhile ago. And I found this video which may be of assistance.
[ame]https://www.youtube.com/watch?v=Ah42w2_XSGo[/ame]
The only problem I would have with the video I posted above, would be the wire flooring. It would allow grass in, and keep predators out, yet if you were to move the tractor with the bunny inside it, the buns paw might be caught between the wire and would hurt the bunny. But I'm sure you would be able to figure it out. Good luck!
(Hopefully you can post some photos when you're done!)