Here are two more spar end fittings that might come in handy for one of your projects.
The photo above shows a 2.9 gram barbed nylon hose connector reducer. It is called a 1/2" fitting on one end and a 3/8" fitting on the other end...based upon the id of tubing that will fit over them. In the photo, however, it is the inside that is being used. The 1/2" end fits loosely over many spars (e.g. Skyshark 400s) and the narrower 3/8" end keeps the fitting from sliding down the spar. The loose fit allows the insertion of, for example, 1/2" webbing. The webbing tightens things up and gives a point to tie things.
The 3 photos above show 1.3 gram nylon barbed hose tees. i think they are called 3/16" fittings but with the barbs they are closer to 1/4" outside diameter. They fit snugly inside Skyshark 400s and give a point to tie things on to the spar. I'm currently testing them on a backpack frame.
Edited to change 1/8 fitting to 3/16. Won't know size for sure until I order some more. Lost package they came in.