I finally figured out what we have been doing wrong! I've heard that many folks make the mistake of constructing their tinfoil hats out of aluminum foil, since tin sheet is not something normally found at the local hardware stores. For the same thickness aluminum is less dense and not nearly as effective as tin, something to keep in mind, maybe a sheet of lead hat would be best?
Yep, tin foil has gotten too expensive and these days just isn't nearly effective enough. I have gone to using surplus WWII polish army helmets for light duty days and WWII german helmets for those days when you really need heavy duty protection.
You still need to pack the space between the helmet edge and your brain box. Twisted and crumpled aluminum foil is a possible gasket material, but may be ineffective.