I've been looking around for an alternative to the pro35 that can be used on my HPX 500 which is a 2/3 in camera. The choices are limited it seems...

Cinevate is preparing new relay lens for B4 mount if that helps, but why are you want adapter for 2/3 anyway?

Because it still makes a HUGE difference! Cinevate and Letus are both working on relays. Great to finally have pro35 competition. I would imagine it won't be that cheap though, the glass needed for the relay needs to be very high quality.