Good idea! I was wondering how to get a hat.
I used Metalized mylar and tested it by putting a radio inside the bag. No radio station got through. I lined my wallet with the mylar.
I made one, many years ago of 1/2 inch EMT and it still has not rusted. Use a program on PC that calculated the frequency and ground each tube to the right pitch. As for the supporting hole I held the tube between forefinger and thumb and found the place where the tube made the best sound and drilled the hole there. I think it was around 22.5 percent of the length from the end.
I used the metalized bag from potato chips as an RF blocker. I tested it by placing a radio in the bag and no RF penetrated the bag. Cut out sheets and placed them inside the wallet next to the outside leather cover.