I've been meaning to reply here for days, but can't get this page to load on the 'puter for love nor money. Well, money might work, if it was used to buy a new computer.
I would suggest something more tube-like than a piece of card - maybe the cardboard from a roll of paper towels? Something about the distance from my eye to the light at the other end seemed to be part of the effect.
I was going to try this myself next time, just to see if it made a difference.