While searching for a non-SFX PSU that could match a M2426 and has more power than a Meanwell RPS-500, I found this 650W AC-DC:
Murata PQU650 Series
Murata PQU650 Datasheet
4.0" x 6.0" x 1.69" (101.6mm x 152.4mm x 42.8mm)
650W active cooling (12V fan header available), 450W convection
active...