That looks like a great option, especially since you already have the fencing mask and it doesn't require buying a whole extra helmet. Hardened leather should be sufficient; if someone has such a flailing blow at the back of your head as to cause injury through a beefy hardened leather shell, something has gone horribly wrong. And $50 doesn't seem high, the leather looks fairly thick. It's probably $25 in materials alone, between the leather and beeswax.