The 'standard' lens tube is a regular 4" female adapter PVC fitting. The business end of the lens tube is made from a regular plug which screws into the female adapter front thread.
Instead of gluing a piece of glass directly to this end cap, I opted to use a 82 to 77 mm step-down ring. This lets me use regular 77 mm filters which I already have, and can use on my lenses when I don't need them on the blimp. A less obvious benefit is that by removing the filter I can get a good grip on the end cap when I need to transfer it between the short and long lens tubes.
Just behind the lens inside the tube, you can see a part of the bolt used to attach the tripod mount to the bottom of the lens tube.