good lord. mihawks sword is frakin insane. I've been thinking on it for a months now, just because i thought for a hot second i might want to try and cosplay him too. Decided against it, but still started planning out his sword XD
I've not really come up with anything spectacular. But yes, for his sword foam would be a no go, it's to thin for that For the blade i would def. go the wooden route. Heres a link for a tutorial on making wooden katana http://chioky.deviantart.com/art/The-Swordmaking-Tutorial-Pt-1-75236042
I know Mihawks blade isn't a katana per se, more like a katana on steroids, but the tutorial has a lot of good tips and how to work a sword out a wood, like what tools you need and such.
Obviously it would be the best to make the hilt of the sword seperate, just for ease of transport. But i haven't really thought of a way to do that yet, and have it connect securely, but i'll put some thought into it ^^
I was also trying to find a way to make the blade of the sword be able to come apart in to pieces, but i can't think of a way to do that and make it not obvious.