The hovercraft calls for a support at 45 degree angles from the platform that the prop mounts to, to the top of the lift duct. The instructions calls for a number of pieces of aluminum bent to appropriate angles to anchor 1/4" plywood to the platform. The edge on the duct would be fiberglassed as the current horizontal support it.
I didn't have the aluminum and it's rather expensive. So I made a support out of 1 1/2" x 3/8" angle and 1/2" conduit.
I think these serve two purposes. The first is to keep the top of the duct from pulling back on acceleration. The other is to stabilize the duct on turns. There are 4 rudders and a trim wing and the tops connect on the top of the duct. If the top of the duct isn't secured, I think it can deform and hit the prop.
I took the prop off so that when I tacked the conduit - I wouldn't damage it. (I did cover everything, but that we a little big to cover.