Double-ended Male/Female Luer Lock Cap. In the image the red parts are printed in FDM while the blue ones are the original injection-molded part.

Fully 3D-printable in FDM in both orientations, no supports required.

Optimized for a 0.4 nozzle, 0.15 layers or less.

WARNING: the seal likely won't be air-tight due to imperfections in the printing process. Liquid can also penetrate between micro-fissures between the layers. You must consider if these limitations can affect your experiment.

Cooling and material is extremely important when printing this part accurately: PETG or PP recommended. Always print multiple copies to ensure the material doesn't overheat while printing.

Using a SLS/SLA process should result in an air-tight part.

A big shootout to this page which provided a good basis for the measurements.