Update March 13, 2018
A care package arrive again, this time with my new set of custom brackets:
The quality is pretty good, but sadly there are several design oversights: Since the thinnest part of the panel is only 1.5mm, using #6-32 screws aren't ideal and several screw holes slipped when I tried to tighten the screw up. Also the top piece is only secured by two screws, which I don't think will be able to handle a radiator's weight.
Next version I will use M3 screws instead of #6-32 and add a support plate between the two pieces.
Also, I got those weird aluminum AIO radiators:
I'm not planning on using them, but they do fit comfortably in the S4M!