All the ones I tried fit like a glove. USB-A Male covers are a little hard to get off due to the near perfect fit, I think it has to do with suction, a little ribbed section on the outside might help with grip. USB-A and C Female plugs fit good, I don't seem the falling out too easily. USB-C Male covers fit good and are slow but decently easy to get off, though I think its because they are smaller as they still have a bit of a suction to them just not as much. But this is better than them being too loose and just falling off.
I don't know about the iphone ones because I don't have an iphone, but a friend does and next time I see him I will ask if he will test them out.
Will certainly look at getting more of each kind in the future (except for the iphone ones as I don't need them). I have a bunch of USB-A based devices that could really use dust covers as I don't use them often. Good for thumb drives that you stick in your pockets so they don't get a bunch of lint in them. And USB-C is becoming more common.
I just carry a small container around with me for putting them in when I go out so that I don't lose them.