I am currently working on a very complex scene for a restaurant where the lighting consists of several hundred lamps. The lamps have directional lighting, so I have to use the "Use Spectron Prims" function for these lights. Nevertheless, this problem does not completely solve, since the Spectron frames do not fit together, which leads to light leakage into the gaps between them.
I don't know what caused the decision to make frames for Spectron light sources in the form of flaps - but could you add a cone frame as an option, which would not allow light to seep through the slits?