Well, I guess that is hard to argue with.
Other prerequisites include
-One working hand and/or foot gripping skills
-Vice or gullible friend with insufficient sense
-Eyeballs or suitable adaptive equipment
Admittedly I envy your hardware store if you can walk right in and find a metal washer with .75" ID and > 1.25" OD. McMaster has very limited options, the cheapest being 98370A036, 0.781" x 1.625" @ $2.80/each plus UPS (you'd need 2).
Further economical alternative mounting adapter solutions:
-Plastic milk, laundry detergent, or other HDPE plastic jug. Sharp scissors. Cut and stack rings until suitable thickness achieved.
-Lucky Charms (has to be Lucky Charms) cereal box, shellac, dull scissors. Cut into rings as above but use shellac to waterproof adapter ring stack.
-Plastic pipe strapping and stapler. Cut and staple pipe strapping to close gap. Attach antenna.
-Coffee can, rocks. Stand in front yard, visibly smash coffee can with rocks until concerned neighbor offers to help. Accept offer.