Another vote for molded rifle case - ensure it's a double shotgun case as then it will be long enough to transport 51" paddles. Canadian tire has them for about $190. Cabelas, etc will also have similar items, just take a tape measure and verify internal dimensions before buying.
http://www.canadiantire.ca/en/pdp/plano-gun-case-0752080p.html#.VZF-JjBVikoDon't get one that has the metal edges as those just get dented and ripped off in the handling process. I had one like that and it lasted about a year then fell apart. It was also cheaper so I guess you get what you pay for. Good quality molded one is where it's at.
A double shotgun case will fit around 6 DB paddles or a couple bent shaft outrigger blades.
In 15+ flights I have not yet had to pay for "oversize" even though it technically is by most airlines definitions.