Bart, thanks for pointing that out. I see that you supplied Norman Koren with a link to a post by Roger Clark on rec.photo.digital describing such a light trap.
That's correct, I got that suggestion from a discussion with Roger.
I understand that the camera looks into the cone from the apex. What should one use for the base of the cone and what is the optimum angle for the cone?
I'm not sure if there is an optimal angle, because it also depends on how directional the light is that enters the cone, and it's dimensions. However, the angle should be narrower than 45 degrees (less is better), in other words the 'depth' must be at least half of the base diameter, but I'd go for at least
as deep as it is wide, if it must remain compact.
It would help if the incident light cannot enter the cone at an angle of more than 45 degrees off-normal, by attaching a glossy black tube that functions as a light shade at the base where the light enters, and that would be at least as deep as its diameter. To keep that light shade more compact and more effective, one could use a short reversed cone with the tip cut off for letting the light in (see attachment).