OK, I didn't take a pic of the end cap, but it is in there.
I grabbed the water tank, and tossed it in there to start checking how things will fit... the tube has not been cut yet but the black line is the probable cut point. Made the 1 x 1 cross brace that will support the front of the tube... I may also lop about 2 inches off the top rails, you may be able to see the marks. I have not decided on that yet.
One of the websites that I tried to buy stainless from fixed a problem I brought to their attention for ordering... so I ordered a piece of 5 inch schedule 5 304 stainless pipe... just got notified they cancelled my order because the didn't have material...
Guess they didn't check stock till I was able to order... Yes I would still prefer stainless because I can weld it in place and add structural strength... the aluminum will have to bolt in to the structure.