And here we have the finished article. The base is just a pill-bottle top, with a couple of washers underneath to give the base some weight. Looks a bit wonky in hindsight, but it works well enough and I'm sure readers can come up with prettier and sturdier alternatives. The adapters could even be screwed straight into a desktop if you prefer.
The main thing about projects like this is to use what you have. Always helped by a sizable odds&sods box in the garage.
Let me know your own approaches to storing pen drives, even if they're better ideas than mine (in fact especially if they're better ideas than mine.)