If you cut the airline tube on a diagonal,
and then drill the hole in the cap 1mm smaller than the tube,
you will have a tight fit with no gas loss.
A simple diffuser is to put a powerhead pump in the tube end of a gravel vac, and a sponge in the wide end, then drill a small hole in the top of the tube at near the powerhead pump end for the CO2 tube outlet. Then the powerhead pump forces the CO2 bubbles through the sponge making them tiny, and they dissolve into the water column of your tank.