The more I looked into it, going super budget didn’t make much sense – a pair of old PS Moves can be had for like $55-60, but for not much more ($120) I was able to get a Dell Visor and that took care of the head tracking and added a much better viewer without the lag of trying to do it with a phone, which will probably save our eyeballs.
Coincidentally, Tilt Brush just went end of line and they open sourced it 5 days ago. There’s already a release:
https://openbrush.itch.io/openbrush
Fired it up on the new (old) Visor tonight for the first time, and the kids loved it. Success!