I was lucky enough to find a local seller who was selling a nice, clean Reflex-Nikkor 500mm f/8 (the 1968 to 1975 design, non AI). It was complete with case and filters, including even the nice (?) leather lens cap. However, I'm reluctant to carry the lens around with this cap since it will fall off if the lens is held vertically. I'd hate to lose it in any case, representing as it does a by-gone era of manufacture. In between uses, I'll just keep the leather cap on the lens, but I would like to find something that would fit this thing to protect it while I'm out shooting and not worrying about losing the leather cap. The outside diameter of the front of the lens is about 91mm. The inside diameter seems to be a little more than 87mm. There are no threads since 39mm filters screw into the back of the lens. I tried to see if a 86mm lens cap would work, but it is just a little too small. The cheapest 87mm lens cap I could find (at Adorama) was an Olympus cap for about $40. Frankly, I'm not even sure that the 87mm would work, and the price is a little steep for buying on speculation. Lens caps can be made by S K Grimes, but for similar costs. If anyone else has faced this and can make a suggestion, I will be grateful. I am planning to make a trip to the grocery store and other emporia to see if some kind of plastic cap for something will work as a makeshift lens protector (a coffee can lid is too large, a kitty litter cap is too small). If I find something I will post, but hoping for suggestions in the mean time. I've attached a picture taken with the lens.