Device to provide elevons?
I like the Du-Bro "V-Tail Mixer" for mixing elevators and ailerons (elevons) on a flying wing or "tail-less" model, such as the Global Cutlass. As the name implies, you can also use it for such things as v-tail aircraft, like the Beechcraft Bonanza to mix rudder and elevator (ruddervators), or to mix flaps and ailerons (flaperons).
What I like about it is the components mount neatly to the top of your servo wheels - no "sliding tray" to fool with or anything to engineer - and because it uses ball links, there is no play or slop in the pushrod connectors or any part of the system.
If you don't have a computer radio (or a radio with a built-in mixer) this is a simple, effective way to go.