I would discourage anyone from using a fiberglass handle in a forging hammer....its very hard on your arm. A fiberglass handle transmits the shock right up the handle and into your hand/arm.....you will become very tired, very quickly.
If your new to forging, my advice is to work with a fairly light weight hammer, no more than 2 1/2lbs. Accuracy and control are the most important factors....if you can't handle the weight of the hammer, your blows are going to be off, and the ability to manipulate the hammer is out the window. Although I do make/sell forging hammers, my recommendation for starting out is to go to a local "junk" store or pawn shop, and find either an old cross peen, or a double faced hammer and if it doesn't have a good hanlde, go to a hardware store and purchase a hickory handle for it.
Your likely going to have reface the hammer......you want it to have a domed face, and the smoother/cleaner the face, the better.