You should be very pleased with this. It is about as fast as one can learn.
"T" dolly, a Covell slapper and a Fournier slapper. I think the plastic hammers are fine as long as they are working well. I tend to have a lot of extra wood an metal hammers that I have picked up over the years so I can re shape them to what I need. Fournier, Covell and Tin Man still sell this stuff.
You also might experiment with the order you do the bends in. Sometimes starting at the tough part that needs a lot of shrinking will give you more space to shrink the extra metal into if you have not already folded the easy bends over. When doing the easy rolls with the slapper you should continue the shrinking as you leave or approach the corners where most of the shrinking is happening. You want to shrink your extra metal over the largest area possible. If you think of it like you need to put darts or pleats in the metal and hammer them flat (making the metal thicker), then you need room to make three or four small darts instead of on big one, which is probably what happened in the first try. You did the right thing by broadening the corners to shrink the metal over a larger area.
Also, you can put darts or tucks in aluminum with a tucking tool.
The tucking tool allows you to make uniform corrugations, which can be hammered down making the metal thicker. It is usually used on bigger panels. But, thinking about how many tucks it would take to shrink the area you are working on will help you not to bite off more than you can chew.
Nicely Done!
David