Compliments of PINHEAD from the forgotten land!
First of all I'd like to thank the guys who made one already and posted pictures. It helped a lot.
I got everything at Home Depot.
The 4" tube is cut to 14". You don't have to get a huge piece of tube, they have 2 foot, precut pieces.
Originally I went with 3" tube, but I couldn't fit my hand in there.
The 4" tube wouldn't fit at first so I had to stack some washers and get a longer 6mm bolt for the front plastics mounting point. That gave me the clearance for the tube.
There are male and female end-caps. I went with the female, the males need an additional coupler to work.
I attached it to the frame with 3 hose clamps. They were for 5"-7" air duct hose. If you rap a tape measure around where you put the hose clamps, it's about 19". I also put a 1/4" piece of rubber between the tube and frame.