For those who operate 99.9 percent CW and don't even keep a microphone connected to their rig and whose rigs use the Kenwood style 8 pin microphone connector (I think it fits the 4 pin type as well but don't have one here to test it), I found a really nice looking SO-239 cover design for 3D printing on Thingiverse.com. https://www.thingiverse.com/thing:4854560

It just so happens to fit the threads on the 8 pin microphone jack used on my Elecraft K3, and many other radios.  I believe it may also fit the 4 pin connectors as well as they seem to have the same outer threads which match those of a PL-259 coax connector shell.  With the proliferation of 3D printers in many ham workshops these days, if you don't have one, it should be relatively easy to find someone who has.

Thanks for posting that, Jim.
I just printed a couple of them, and can confirm it fits the 4 pin mic jack, as well as the 8-pins on my K2 and TS-430S, and of course, the SO-239.

I found a BNC cap on Thingiverse, will report back after I print one to try out.

Jim, N5IB

I can also now confirm that

https://www.thingiverse.com/thing:4298479

is a nicely fitting BNC connector cap to 3D print
Have three of them on the K2 now

Jim, N5IB

Just printed the first one of several BNC covers. Nice and it fit perfectly right off the bat. I have several places these will come in very handy.

W0EB