Thanks. I'm reading on a phone. A cheap Samsung. It's very hard for me tell the difference between the normal text and the hyperlink. Back in the good old days, hyperlinks were bold and underlined (back then my sight didn't need that help).
I may be asking after the wrong thing, your Google search was less successful than mine ‐ everything is hollow. I was looking for something solid to block the hole.
I found things that looked like (ic) engine valves. Nylon and long enough to be clamped by the conductor screws. Ideal, but £30 and on ebay. Ebay search "Henley block blanks". These are someone's home 3d printed project.
The next best thing was rubber plugs that would "snap" in place. Ebay "White Snap-on Hole Plugs Silicone Blanking End Caps Pipe Tube Inserts 2.5mm~14mm". Special order from China.
I thought this may have been a common requirement, meaning I just needed to know the name to buy an assorted box cheaply.
Looks like it is not a common request (just me being lazy again).
Thanks for looking.