Is this a mistake Part 2

I have the lower beds running on the auto feed/drain system, now to get the upper beds going. I need to adapt my 3/8" OD lines for the drains. Could use my Wire Gromments (expensive), yet wondering if I bought some rubber grommets it would be a way to go (seems cheaper for the same seal).

So, on that thought, will 3/8" rubber grommets work or should I be looking at 5/16" and be sure they pinch/seal? I can always heat the tubing and fit it.