If you don't care about originality, and own a lathe, a nice fan can be made out of square stock bored for the knurled screw that holds it to the shaft. Then add FOUR fan blades (one on each side, they don't need to radiate from the exact center) and balance it between centers on the lathe. Use trial and error to figure out the speed you are going to end up with. I guess I'm just happy with a 3/4 second fan even if it is an odd speed. I've tried adding weight thinking that would even out the spin. It does, but it takes some time to get up to speed. There is probably the exact right combo in there somewhere. I think a real improvement would be a fan that has a bearing support on the outside end of the shaft. That would prevent wobble.

You can add pieces of flat metal ,foam board, or mat board to a fan to slow it down for long exposures. I have one somewhere in my collection that is so big the bottom sides are beveled to prevent it from hitting the tripod legs. It's cool to watch it spin in slow motion and you could read a book during the exposure. Needless to say it isn't used much but looks good when you are on a stage demonstrating the cirkut for that big ego rush we all need to get once in a while.

The way the original fans were made is a bit complicated to keep the knurled screw from falling out. Not a problem if you have dedicated machines to make thousands of them but a pain to duplicate one or two and make money. I've made them just to prove I could do it, but when it takes all day to make something they are hard to sell. Again, if the demand were high, a person could make the tooling needed to mass produce the parts. We know that ain't gonna happen in this day and age but can dream.