I made a different version of this mounted further back in the bed. I used a 2x10, and bolted 1 1/2" EMT to the board using EMT one hole straps. Then I slid the flagpole (1 1/4" EMT) into the base and secured it. The assembly is tested to 80mph without any issues. The flag however, doesn't fare well at 80mph.




Here is my revision:

Mock-up in MS Paint



In the Bed



Flying the flag

 

Here is my flag mount that I did with a few things laying around the house. Works great. I've tested it out in 70 mph winds and rode 50 miles at 65 mph the whole time and it didn't break and only frayed the flag a little bit.