When I first got interested in photography, even the cheapest lenses and accessories came with a case. Frequently leather and pretty good quality too.
Then we went through a period where lens cases were sold separately for most lenses. Fair enough.
These days, manufacturers can’t be bothered. Lenses costing thousands of dollars come with a loose-fitting pouch, held closed (maybe) by drawstrings. These things offer almost no protection from bumps and being open at one end, they obviously don’t keep dust out.
Sometimes, you can pick up an old case on eBay but you’ll have to search and probably pay too much. There are some Chinese neoprene pouches but they’re more like envelopes. They’re flat, and still open when you fold the flap over (see photos.)
OpTech, the folks who make the neoprene straps have come up with a good solution. Neoprene pouches in various sizes. They’re actually two pieces connected by a small, sewn hinge. They weigh nothing and they work great.
Note that depending on your lens, the fit will not necessarily be snug so when the “cap” is off, the lens can slide out of the pouch if you flip the top part open and turn the pouch over. Of course, this could be a good thing if you need to get your lens quickly.
OpTech makes these in lots of sizes, including some very small ones. I even kept the ones that weren’t a perfect fit because they’re the kind of thing that’s handy to have around.