Examined my Ritter Photon closely. The short length of tubing kinda resembles the 1/4-in. black micro tubing I use for drip irrigation. Put a cheap micrometer on my Photon tube...~25-in. x .18-in (metric? 6.5mm OD x 4.5mm ID). Don't trust my measurements.

Wonder if the tubing is press-fit, or held in place with a dab of epoxy? I like your conversion idea, but would you really need heat shrink tubing?