Just a thought, and havent done it yet, but I figured I could make one for my '61 by using some of that thin insulated material LMC sells (part# 38-2829) $30.00 for a 4' x6" roll. Make a cardboard (so you can fold it up) template that fits your windshield, and glue the stuff on with the foil side facing out of course. Your '64 windshield is flatter than mine, so should be a piece of cake.
Before I buy from LMC, I'm going to check if there's something similar for less $$$ at the local Lowe's/Home Depot type places.