The added length can be done in the area of the seat and the contour of the body will not be altered beyond the appreciation of those that care.
This is a good place to make the finish skin from steel sheet that will not only make it much stronger, but help keep all the important juices on their side of the fence. and require them to go outside in the wind before bothering the neighbor.
That area can be as permanent as you make anything and it should never be in he way of any access.
Moisture that results in corrosion from the inside , and cracks are the most difficult to keep track of.
During assembly. the tubular frame components can all be interconnected with a tiny hole at the intersections and be pressurized with dry nitrogen.
You will need two !/8"npt bungs and one fitted with a screw in Schrader valve and the other with a cheap deal gauge like on a $10 home fire bottle.
Now with a hand spray bottle of soap and a rag it will clean outside stuff as you search for bubbles that indicate cracks you might otherwise never see.
If the gauge drops, it is probably a good time to get the soap and the rag.
It works the best and still looks cool.