From the sample picture supplied, I would surmise that what is happening is:
- The snoot is relatively long compared with the overall size vis-á-vis entry/exit diameters.
- The small exit hole is working like the 'lens' on a pinhole camera. You have a slightly defocussed image of the grid projected onto the wall.
- The black spot results from the above 'pinhole' effect showing the lack of effective light from the centre of the source. It all boils down to the exit hole being so small.
This is a
theory, you can easily prove/disprove it by dropping the shutter speed down to a point where the modelling light actually is contributing to the exposure. If the black spot lightens or disappears then that is what is causing the problem - you're getting the effect of a projector instead of a snoot.
An alternative would be to remove the snoot, use black paper as a cylinder between Esprit and (hand-held in place) snoot. By extending the body (NOT the small end) of the snoot in this manner, you should be able to see if my conjecture above is proven.
Yes, the Bowens genuine snoot is silly money - it's a factor of low volume production - versus the one shown (generic Chinese - one size fits all but not very well??). But the Bowens does at least work as you hope. A snoot is nothing more than a frustum of a cone - the trick lies in balancing the length with the exit diameter. All the "maker's own" snoots on which I can find info have relatively larger exit holes than the one shown by OP. There might just be a sound reason behind this!