Not sure if anyone else has sussed this out yet. Got myself a cheap set of rails off of Ebay and by reversing the bracket that attaches to the base plate and using a longer screw it works great with my HV20/Extreme setup. Placed two strips of velcro along the extreme where they make contact with the rails to act as a cushion and stop any wear.

Haven't seen anything else this compact.

I got the rails for £60 inc delivery.