It was these ones for the cranks, in the smaller size.
As you can see from the pics I had to trim them down quite a bit to fit the Sram dub cranks.

Is the bung just held in place by friction?

Yes. It’s a really tight fit though, and the pump is really light so there’s very little force trying to move the bung.
As I said I rode it hard at Bikepark Wales for a day and it didn’t move a mm, so I am confident it won’t move.
Don’t think there’s any risk of the pump jamming the front wheel, even if it did fall out.