I am not at all surprised that I didn't even know there was a Rainbow Lake up past Detroit. There are dozens of beautiful lakes up in that area. The only real info I found for you is that once you pass Idanha,turn right onto Coopers Ridge Road. If you have a rig that can withstand a gravel road challenge,make a quick left to take a shortcut. Please come back and let us know what to expect if any of us get up there. Pictures would be ideal. Good luck and great fishing.