The Cinefoil solution seems promising. The light coming from the back makes the qualities hard to distinguish. I would need to see more shadows and fall-off.
Here is an interesting thread about trying to make a DIY solution:http://forum.luminous-landscape.com/index.php?topic=41247
I think the bottom line on this is eliminating reflections (black interior and some distance of run from the flash tube to eliminate stray light which might bounce off of other surfaces--walls, ceiling, floor or ground, etc) and exposing as little area of the flash tube to make it as much of a point source as possible (why the strobe unit is at a 90 degree angle. Presumably a differently shaped flash tube would benefit from a different orientation).
This leads me to speculate that this might be a viable solution (although aimed at a different system):https://www.amazon.com/Godox-Snoot-Honeycomb-Speedlite-Flash/dp/B00F15063U
The tube on this unit (Godox 180/360) is shaped like a test tube, so the orientation from the snoot makes it quite a small light source. Black paint makes it non-reflective and the grid presumably further restricts reflections due to spread. I'm guessing the light would have to be backed up a bit to cover much area
. But it makes me think that with a cylindrical (test tube shaped) flash, a larger snoot connected to a reflector painted black (without the built-in grid?) might work...