I still cannot post photos, but there should be a pretty beefy piece of aluminum running vertically along the left and right vertical edges of the rollup door. Our entire back section is held by eight bolts (4 to a side) through those two verticals. You could screw in two links of quarter inch angle aluminum running vertically to give you a flat face to which you can attach the aluminum sheet. Then, just use something like top hat or square aluminum tube to reinforce your sheet from the outside. This is where I would use rivets instead of self tapping screws. You can Easley frame in the door and provide additional structural support for accessories. I would probably try to have a large propane bottles supported by the old structure other than hanging entirely off of the new unwelded section.

SM1, I like the sound of it. Do you have a picture? If you cannot post pictures yet see my prev post on a workaround.
Are you saying rivet the aluminum top hat or square channel together to form the wall, or does it have to be welded, and then only rivet the 1/8th aluminum sheet to it?
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Blakkmoon, would you frame out a wall in treated 2×4 's? and then screw aluminum to the outside, and plywood and FRP on the inside? (like pic #2 above on my opening post?).. �assuming you go with a steel slab door.
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