Got my secondary flies and shafts out today and then experimented with several different plug fabrications. Here's what I settled on. These trim fasteners came from a local auto parts store. They had lots to choose from. Other types would have worked, too. These were almost 7mm thick, but when pressed into the 5mm shaft holes, the fins collapse. This makes for a very snug fit. Just to make sure the seal is airtight, I wrapped them in Teflon thread-sealing tape. The end result is very tidy, as you can see. They can be pried out with a screwdriver, but they are quite secure. These plugs were slightly longer than the holes were deep, so I just cut them a little shorter before installation.
This is a cheap, easy, readily available alternative to the other plug fabrication ideas presented elsewhere.