We made one that goes in the trailer hitch and the pole is just black pipe it screws in and out I had to lengthen the pole to 8.5 feet so it would flag correctly. You don't want to go much faster then 40 with it other wise it destroys the flag.
Here is a pic of how I had the flag mounted on the highlift jack on my Samurai.
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If you are going to run a flag a lot you should check into getting a quality made one so it doesn't fray on you and a smaller one. I've had a few different ones and this one from West Marine has held up the best. This is the same flag I ran for years on my Samurai and now it has been moved to the Jeep and still looks great.
Here is the mount I made yesterday.
Google CB antenna flag ... I buy them in bulk a dozen at a time
They have a long cloth sleeve you thread down the antenna
Last about a year unless I tear one off on the trail
Burn the worn out ones in campfires on the trail with proper ceremony