You could get the female adapter you have found, and put a 3/8-24 all-thread in the middle.
I've been making 3/8x24tpi all-thread studs by cutting the heads off stainless bolts I bought at the hardware store. A 3/8x24 die will help clean up the threads at the place you cut off the head. But I've found it's much easier to order a handful of them from High Sierra Online (HeathTech). They sell them online for a couple of bucks--very easy and convenient, and they've got a bundle of other mobile antenna parts, so I order from them fairly regularly.
Michael, pardon my addressing YOUR question in the middle of the other question, but I love HF mobile, it's a world of fun, and I also love my 817, which is a nifty little radio, no doubt about it. But as N5MOA said, hamsticks or their clones might be a much better choice for starting out. Running mobile HF is tough even with 100watts; if you're going to be trying it at 5 watts, you want to get as much antenna efficiency as you can! N5MOA is right that hamsticks are pretty efficient and pretty cost-effective for mobile work on the higher bands, and 20-17-15 are all great bands for mobile work. One more vote for trying that first!
I'm using a different mobile system now (you can see it if you look me up on qrz.com and then follow the "mobile" link), but I used hamsticks for years from the car, and had a lot of fun with them, at very little cost!
73 GL! --ken ac4rd