covering sides of LEDS without adding heat
Heatshrink varies from 25% to 50% to 66% reduction in diameter depending on type.  LED's are extremely sensitive to damage risk with soldering > 3 seconds on the leads but no problem with a heat gun on the tubing.

This is a perfect solution to your problem and you can reduce the beam exit angle by the length of the tube. If it shrinks to say 3mm, you may have to align it perfectly and reinforce with PolyUrethane, PU (e.g. sub-floor adhesive) so it does not move easily, yet not brittle.

If you know trig, you can compute the length of the tube for the cone angle you need, so that alignment is not critical yet blocks interference.
  A PhotoDiode, PD and high gain current sense amplifier is far better with a daylight blocking filter as they are very accurate with 0.4mA/mW but PT's have high gain but very inaccurate with >50% hFE tolerance.  But you can always trim your gain to suit the gap.

Usually a 1500W heat gun with low air speed is used with a curl adapter to circle around the tube which is hot enough to melt solder at shorter ranges.  I have also use a lighter with care, and a hair dryer on low speed.