Sure there is a buddy box! and it works great! see the attached photos. One photo with the box unassembled, and one assembled and ready to go :}
The wooden handle actually says "Control Line Buddy Box". Idea certainly not original with me, but basically:

a) setup the handle as shown. I go out and start the plane flying using the yellow handle, while guest stays in the pilot circle or safely on the sidelines.
b) i move my hand out to the wooden (inner handle) to fly. can do this just with fingers, or two handed.
c) i take the plane up high and have them scamper into the circle, and stand off my left shoulder (or stand up if they are in the circle) and face the plane.
d) I put my hand on their shoulder and we start rotating. generally they pivot in place and i run around them flying.
e) in a few laps you can tell if they are going to go wobbly or stand up ok. if they wobble and fall, you just fly, all is ok.
f) if they are doing ok, they reach up and take the yellow handle with their right hand. they can get in the safety thong if you want them to, but generally i don't have them do that
g) i give them more and more tension on the controls as they get a feel for it. i can coach them thru manuvers, as i drive on the inner handle, they follow behind on the outer handle.
h) if they are doing ok still, i can even drop down and let them fly, and jump up and get the handle if i need to.

i have trained several people to fly, from kids as young as 6 (my back aches from reaching down so much) to guys as big as 6'4" and >250lbs (i run alot, takes alot of steps to go around a dude that big) but i can say that crashes are pretty rare.

if folks think this is an interesting topic, let me know and i will document it a bit better, including safety tips and take a video of it.

r.