UniFoxoS, my description probably lacks the required detail.
With PAPI there are only two zones in the beam, the upper (white) and lower (red) which are projected from the white/red ‘slide’. The horizontal dividing line (constructed on the slide) gives a sharp transition which should give high precision. The beam width of PAPI is whatever the slide / lens / casing allows.
The problem with VASI (no lens system) is that the division between white and red is very diffuse and offers little accuracy as well as lower beam power.
Try Google for more info; you will only need a single tube system.
IMHO the laser pointer option would be much simpler, although this has no lateral spread.