I'm seeking a decent lens cap to use with a 77mm slim filter. A slim filter is used with super wide angle lenses to avoid vignetting but does not have front threads like a regular filter, so a regular lens cap does not work. B+W does make a push-on cap for slim filters, but many like me have found it falls off much too easily. It's odd they did not design it a little better with more grip. I've also tried a neoprene lens hood, but the problem with that is the neoprene touches the glass when on and more so while putting it on or taking it off.
Seems like a tennis ball can lid or Pringles can lid would work perfectly if made in the right size. Film enlarger lenses and binoculars come with such lens caps, so surely there could be one for common filter sizes. Alternatively, a regular lens cap that grips from the outside rather than inside would work too. Anyone know of such a product or source?