The HamBone

Telescopic antennas - particularly for QRP - often end in male M10 threads.
Here's just one 18' example from Amazon:
B0DCMTX7QD

And here is a common mount for portable / mobile antennas. Great but they all seem to have a 3/8" thread.
B0765QHQMZ

My kludge of a solution is use this adapter:
B0BV6KGGX3

The antenna, mount are sturdy, but this adapter has short threads and is made of aluminum alloy. Maybe not so strong?

This works, but what's a better idea?

Amazon ASIN numbers are with each photo.

Well, the adapter above arrived and doesn't fit.
The package contained two well made aluminum pieces, with a good m10 fitting for the antenna, but what should be a 3/8" fitting turned out to be m10 too. Returned for refund.

I have found some other possible sources for an adapter, and will post again when I find something that works.

The plentiful supply of 18' m10 telescopic antennas make the adapter hunt all worthwhile!

A partial solution:
Turns out there's an automotive brake line union with 3/8" female one side and m10 female the other, from several suppliers.
It fits the 3/8" side well, but the m10 is the right size but not best screw thread. Works, but still looking for a more robust solution.

Here's the solution for HF 80-6 telescopic antenna for portable QRP!

It seems that this Stainless Steel 18' telescopic antenna works well, which will need this adapter to make any metric m10 threaded antenna fit a US 3/8" mount strongly, securely and inexpensively on a 3/8" to SO-239 adapter which you put on the mag mount or whatever of your choice.

-But not quite done yet, as you probably want an adjustable loading coil. Hint: maybe a little tight on delivery, so I squeezed the contact spring a little and applied a very little light lubrication. Seems good now. and if you insist on trying 80m you'll need add extra inductance in It's easy to add and subtract inductance by these sturdy m10 interchangeable parts.

-So there's your stainless vertical with adjustable loading complete for under $100.

Can you do it with Chameleon and Wolf River? Yes, very nice, but be prepared for a multiple price difference.

Whichever you decide, it seems that these verticals work even better if you run a clip from the outside of the mag mount SO-239 to the vehicle metal, as counterpoise / ground plane.