Very low tech: Pour a layer of paraffin wax over the top of the liquid ingredients. The wax will harden into an airtight seal with no gas between it and the liquid. To use, push on one side of the seal to dislodge it from the sides of the container, remove the wax "puck," pour off the liquid you need, then reseal with a new layer of hot wax.
Advantages: Super low tech. Use any container that is compatible with the ingredients. No machining or other precision manufacturing needed. Irregularly or roughly shaped containers will work fine. A sealed container can be tipped or inverted without harm.
Disadvantages: Super low tech (i.e. not as cool a solution). Slower to use than e.g. a syringe. Need to have hot wax ready before use, to avoid having the contents exposed to air for too long. Ingredients need to be tolerant of heat of melted wax (approx. 50-60 degrees C).
This is how jams and jellies are sealed when canning them at home, except that jams and jellies are not typically resealed between uses.
You can also use beeswax for this. It is even more low-tech. The melting point of beeswax is about 63 degrees C, so the same caution applies about making sure the ingredients can take the heat.