Hi, long time lurker, first time poster. I got my first Pax last July, and bought my second one the next day. Love the unit. I have read this entire thread since last June, but haven't had anything to add.
But now I think I have some useful info. I found something that works great as a Pax cap. Mentos gum comes in a container that looks like a shorter Pax. The container has a lid that can be taken off the container. There is a collar connected to the lid that needs to be cut off. This lid fits perfectly on the mouthpiece end of the Pax. It snaps onto the end snugly, and stays in place unless removed. There is a ridge of plastic on the inside of the cap. This needs to be cut off (I used an exacto knife) in order for the cap to fit snugly on the oven end of the Pax.
This picture shows the container, the cap & collar removed from the container, the lid with the collar removed (with the ridge of plastic still in place), and a Pax with a cap on each end.
This shows one cap with the ridge intact (fits the mouthpiece end), one with the ridge shaved off (fits the oven end). The better you shave the ridge, the more snug the cap can fit the oven end.
Hope people find the info useful. The caps fit very snugly, and while not perfect, they do contain the smell very well. I leave a Pax in one of my jackets a lot, and my wife tells me that the jacket starts to stink after a while. Since I've been using the caps, it no longer smells to her.