At long last, I've finally made my HP WMR headset comfortable. I got my hands on an HTC Vive DAS and I managed to attach it to my WMR headset. As far as I know this should also work for at least the Acer and Lenovo WMR headsets as well.

3D prints
Top pieces:
https://www.thingiverse.com/thing:332...
Side piece:
https://www.thingiverse.com/thing:328...