Hello! The roof element of the 10" velux rigid sun tunnel has already been fitted over the new stairs to the loft conversion. My part is to fit the internal sections. I have fitted the upper elbow to the roof element. The lower elbow will fit into the ceiling disc when it is fitted into a new ceiling - still to be attached. Problem: the 24" extension piece is too long to fit betwen the two elbows. I tried cutting the extension piece to about 20" but it is very difficult. The two elbows might just overlap each other - should I open one of the rivets to make them fit into each other?
Just looking at your post. Never use duct tape for installing sun tunnels. Highly reflective tape should be supplied with all decent products. Some areas will expose this tape within the tunnel, so it needs to reflect any light downwards. Duct tape has no reflective values.