I use my RKE to control my ATO and it's very simple and quite effective. I trust it more than I did my JBJ ATO. You'll need a float kit and I believe the switch device.
I think you're missing the point. You already have the head unit with your RKL. Just purchase the add on pieces from Digital Aquatics or through eBay and you're set to go. Other members here might even have the parts needed to add to your RKL. Go to