I have some Pony bamboo interchangeable circular needles. They come in 2 attachment sizes, brass and black painted. I like them because they have good pointy tips, and feel nice in my hand. Also they are easy to get hold of and are a good price. I find the joins are reasonably smooth, and they don't come undone easily when in use.
I decided I needed some kind of storage for them, and when I was in the stationery store I noticed some wetsuit fabric pencil cases. Great, I thought, I don't need to make the complete bag, just adapt these. I bought 2, one in pink and and one in blue.
I sewed some dividing lines to make pockets for pairs of tips, with a wider pocket for the cables at the end where the zipper pull sits when opened. I also made some horizontal pockets above the zip for the cable joiners and end stops. The brass ones are in the pink, the black painted ones are in the blue.