With screw-top lava lamps I have topped up small liquid drops using distilled water then wrapped the glass threads with PTFE tape before capping.
With bottle cap globes I tighten the cap to the bottle with pliers before wrapping the cap where it meets the globe with self amalgamating rubber tape.
(I have found it best to place the cap back on the bottles/globes when the wax/liquid is hot)
Hope that helps.
Hi Steffan and welcome aboard 'Star Ship OG.'
The level is not that bad, I would top up with just DI water, done this on lamps of my own when the level has been much lower than the lamp in your photo.
Good luck and keep us posted.