This is a very simple mounting system for an outdoors light bar to mount over my desk, with adjustable distance from the wall by just untightening the screws on the slides.
Hope this is useful to someone!
This requires no supports apart from the overhung screw tabs on the wall piece, unless you have some absolutely freaking amazing bridging, I recommend using tree supports, works a charm.

It does require at least 6 short m2 self tapping screws and 4 wall screws, for the tightening screws I recommend torx screws if you plan to move these a lot, I just find them more manageable than cross screws.