Is it true that Hitler's hat was custom built to serve as a helmet, lined with metal and weighing several pounds?

According to the website I found, no it wasn't. Hitler's hat was no more than a black top hat, nothing necessarily unique about it's build.

http://www.jeffkrulik.com/hitlershat/story.html

Here's a Youtube video with an interview with the man who found his hat (There was actually a movie made on the story of it's finding):

http://www.youtube.com/watch?v=K41ZxYTqd_I

The hat in question may be the one used with his military uniform. I once saw a documentary on Hitler which went into some length about his hat being designed as a helmet and consequently being very heavy. I haven't heard anything about it before or since and have often wondered if it was true.

Oh sorry about that, then you probably mean his visor cap. I'm trying to find another resource on that. Here is a forum discussing that specific topic, according to what others say, it was probably custom made and a guard of Hitler's said he had been built to be bulletproof.
wehrmacht-awards.com/forums/showthread.php?t=366561 [no longer exists]

"It is generally thought among conspirators to guard against attempts of assassinations, Hitler would have worn a bulletproof waistcoat and cap". This is taken from the below link (Google books), it talks a bit about Hitler's cap (be careful with scrolling the page)
http://books.google.com/books?id=212hoVrF3B4C&pg=RA1-PA278&lpg=RA1-PA278