Did you try a Snowboard travel bag? .
It is too wide for a normal Snowboard travel bag.
I use a cabrinha bag (the big one) with wheel.
I have been honest with the people at the checkin. I just say
that is a "Kite board" and if they do not understand immediately what I am speaking about, I add quickly that it is like a snowboard just a bit wider.
I am carfull to avoid any words like "surf", "windsurf". One time they look in a kind of document that listed the type of fret and decided finally that it came under the sction of "snowboard " and I did not have to pay for any extra.
If I understand well, they are afraid to carry something fragile as if its breaks during the flight, they are responsible.