At Waterloo in June 2015 I will be portraying a major of the 1st Foot (Royal Scots).
I have been working on the bicorne hat which I purchased on Ebay, and have achieved this:
But apart from the Waterloo event in 2005 when temperatures reached 38c (100f), every reenactment I have done in Belgium has been wet and muddy, so I have made a cover to protect the glorious appearance of the headgear.
I searched the internet and every reference book I own for a pattern for the waterproof cover to no avail, so finally had to design my own. I find it amazing that although there are so many references to covers for headgear in this period there is little evidence as to how they were constructed.
General Whiskers' semi-historical wanderings 
Actually I think Brassey’s has a representation in their British Book. I’ll see what I find.