Shortly after customers purchase a ReefKeeper aquarium controller, they'll usually call or email us back a week later and ask us how to mount it.
Many hobbyists keep it simple and use good old-fashion screws to mount the controller to their stand. If you don't mount the controller in the perfect spot, you are, in a word, screwed. You can always remove it, but then you'll have to cover up or disguise the screw holes.
We prefer something a little less permanent in case we change our minds--or tanks, for that matter.
Using Velcro to stick the controller to the side of your aquarium stand is another popular choice. We actually have a RKL here in the office mounted that way. Velcro is nice because you can easily remove the controller to fiddle with it. Of course, you'll always have pieces of Velcro affixed to your controller and stand.
We came up with another solution here in the office: using a suction cup holder designed to hold portable devices in automobiles. Turns out those inexpensive cell phone and GPS holders work great for cradling your RKL or RKE.
The following photographs were shot with, you guessed it, our mobile devices, so we apologize for the meh quality. The model used in these photographs is the Fosmon Windshield Car Mount Holder available at Amazon.com for $9.99.