How to I fit a drawer into this cabinet? (Kitchen - no side panel)
I'm sure someone with better google-fu will be able to chime in with a quick link, but I too have exhausted all my usual suspects looking for what I would call a "rear mount bracket" (For example, Blum part # 295.3750.01, but only on the Blum US site.) 

Anyway, a bit of bodging would have you place an L bracket against the back panel (with the caution that the back might be very thin/flimsy) and catch the drawer runner. 

A long enough L bracket could go from the underside of the worktop to the runner, but you'd have to be careful that the bolt attaching the runner and the bracket doesn't interfere with the operation of the drawer.

Alternately, put a section of wood vertically beside the drawer and flush to the edge of the opening to mimic a cabinet side. You could use a couple of L brackets to hang this from the underside of the worktop and it should be more than solid enough to carry a drawer.