As was stated above, some film canister caps make great rear caps for the 5cm and 3.5cm Nikkors.
I used a Kodak film canister cap. The Kodak canister with the gray cap. Usually Plus-X or 125PX was packaged in it. A bunch of color films used this canister too.
I took a razor blade and removed the inner lip of the cap then tested it out on the lens and it fit snugly. Since it didn't lock, I always kept a rubber band around the lens to hold the cap on while it was rolling around in the bag.
Another option is to get a few caps from Jupiter lenses made for Contax mount. They fit snugly and are made of a somewhat pliable rubber which really sticks to the lens very well and protects more than just the rear element.
Phil Forrest