I took a 10 foot steel conduit, cut it in half to make two 5 foot sections.
Measured out how far apart the eyelets on the flag are.
Drilled holes and installed some 1/4" eyelets through the conduit. (I used nylocks to secure them)
Inserted them into brackets on truck, and marked just below the upper bracket.
Drilled small hole through conduit for a retaining pin so they don't come out if I drive faster.
Painted them black.
Attached flag with carabiner.
Can be installed or removed in less than a minute for both of them.
edit: I use Annin ToughTex flags. They hold up well to 60mph and above for a few hundred miles before they start to show signs of fraying/unraveling at the end, then they get hung on the house.
At over 65mph the conduit does start to bend a little bit.
I'd much rather have the conduit bending than the brackets. Not a bad idea to leave a weak link like this just in case you forget to remove them before going into the parking garage; conduit is cheap and easy to get