I wrote instructions for the one in my avatar...you cand find those here:
I would suggest either of the Wilton ball pans...the sports ball for a smaller helmet (my avatar), or the soccer ball for a larger sized one (photo below). These give you the perfect dome shape on top...I would never consider carving that from a tower of cakes.
(The soccer ball pan is tricky as far as baking...I had to experiment after getting help here at CC. If you think you'll need to know about that...just say the word)
I agree, the face mask can be the toughest part...the second, larger helmet I made ended up having no mask....the black one I made basically came apart at the last minute. I made it about a week before, so all the individual pieces were rock hard, but they wouldn't stay together...the were too heavy to support each other. I tried every type of glue I had, toothpicks, straight pins...I could not repair it.
I'm now experimenting with plastic tubing, so if I ever do a large sized again, I'll have a reliable mask.
Even though it was for my nephew, and he didn't mind at all, I still felt terrible....it looked more like a space helmet to me (and really flat in the photos...in person it didn't at all). I liked the Chargers jersey cakeboard that I made, that came out ok.
The round masonite board underneath was just for transporting...that was removed and the jersey was put on a green field tablecloth purchased here: http://www.profootballparties.com/Store_ViewProdDetail.asp?ProdID=11498&Cat=328