AC guinea pig...

Cool. We love photos and custom mod's. I'll just caution that you need to consider all the forces on the compressor. To fix an object in space requires at least 6 reactions - 3 force directions and 3 moments (think pitch, yaw, roll in an airplane), so insure your brackets are stiff to oppose all such forces. Even though the belt only applies a single force, vibrations can setup oscillations in other directions which can cause the metal to oscillate and crack from "metal fatigue". A guy on my M-B forums sold custom Sanden brackets for my 1985 diesel engine. I installed them and worked fine at first, but I found cracks after 2 years and eventually had to give up since they cracked to pieces, and he was using 1/2" thick steel plate. Probably didn't help that I cut a large hole like you show to reduce weight. Also, diesel engines shake more, so bad that even the factory air filter brackets usually crack. You can find many pictures of rodders mounting Sanden compressors using many swivel-end rods (like tie-rods), with the compressor mostly up in the air on "legs". This can work if they have enough to make it stiff to all 6 possible loads.