I would take one of those small containers used for disposable ear plugs, cut a peice of foam to secure the bulb(s) and attach to a lanyard.
I think Remington sells a small screwtop container w/ earplugs.
I recently found a small waterproof container at REI that is a perfect fit for 3 PR bulbs and very flat so it carries comfortably in my pocket. Some PR bulbs (such as from Mag) are larger than others, and you may find that 2 is a better fit than 3. Add a small pice of cotton or foam between the bulbs to prevent rattling. These are manufactured by "LA Cons" (USA) which also makes similar containers for ear plugs as mentioned above. The lid snaps on rather than screwing.
The product is called "1/4-oz. Poly Container $0.30 Item 605887" and can be seen at this link:
I have not found a convenient way to attach this particular container to a lanyard except by punching a hole in the container, which would make it non-waterproof. Some of the larger containers have more of a loop of plastic between the body and lid that could be used as an attachment point.
BTW, a warning: The vertical clearance inside the 1/4 oz. container for the PR bulbs is very minimal, like measurable in hundredths of an inch, and being plastic, the container is slightly flexible. It would be possible to break the bulbs inside the container with rough use, such as stepping on the container or maybe even sitting on it. It should be fine for normal use, though. If you need something really bomb-proof, go for a larger container such as the 1 oz container and add more padding.
These are also useful as pill bottles and containers for small items like safety pins, ear plugs, etc. so if you get one, get several.