Well I thought it over in my sleep last night and came up with a way to secure the elbows without using epoxy. They were not very secure with two o-rings but I got the bright idea to use a section of hose on one side and the o-ring on the other. This holds them nice and snug but they can rotate. I like this better because if I ever change instruments I can reconfigure the orientation of the hoses.
After deciding this I stilled and riveted on the nut plates that will hold the base of the duct and I riveted the doubler that the elbows go though. The last thing I did before it was time to run errands, which killed the rest of my day, was to epoxy the duct to its base. Tomorrow I can mount the duct in place which will allow me to drill the mounting holes for the fan motor.
The fiberglass base has the holes enlarged to go around the o-rings on this side
