If you're fitting the pipe over the end of the funnel, finding one that has a barb on it, or adding a tiny hose clamp might be something to think about.

If the grocery store/essential oils/syringe ideas don't pan out, you might look into aquarium or watercooling hardware.

That said: https://www.amazon.com/Syringes-Dispensing-Scientific-Watering-Refilling/dp/B0822SDKS9