One issue I have with HDPlex / PicoPSU solutions is how the AC-DC component negates any potential space saving. Perhaps this will be useful?
https://www.emea.lambda.tdk.com/uk/pfh500f — 500W, 28V DC, 101.6 × 61.0 × 13.3 mm.
https://www.emea.lambda.tdk.com/uk/pfh500f — 500W, 28V DC, 101.6 × 61.0 × 13.3 mm.