I just used the HDx35 PL to B4 adapter last week on my Alexa. I used it with a canon 4.7 x 11 with 2x extender. I just had a screw in polarizer on lens shade due to the fact that once I put adapter and lens on, I didn't have long enough 15mm rods for matte box. The 2.5 light loss plus my polarizer helped with exterior exposure. The 2.5 magnification also greatly increased range of focal range for documentary style and handheld. Don't forget a cable is needed to power the 2/3" lens and trigger to record. I had Y cable that provided 12V power and one that triggered Alexa. The 2.5 x magnification of my lens at 47mm with 2x extender would be around a 235 mm on a PL lens (and shooting 1.
I have to say the bokeh is far from the Alura lens. PL glass is far superior but I knew I was still going to beautiful Alexa dynamic range and codec.
If you're looking for length a standard 7.8x21 with 2x extender would give 410mm without the the 2x extender.
Not cheap but for documentary work a viable option.