I haven't built my TD yet and will not be using aluminum on it, but from my background in aircraft design..... even 0.04" thick is extremely strong depending on the alloy and temper used. Aluminum has been the holy grail of materials in the aircraft industry for the past 60-70 years, and every ounce of weight that is not needed in the aircraft is attacked vigorously.
If you are just floating the aluminum skin on, go with a 2000 series (2024 for example) or a 6000 series clad aluminum in the "O" (untempered) condition and get it nice and thin (0.025 or 0.032). A 7000 series alloy would work great too, but that's overkill for a TD. If you are going to glue the sheet to the wood or fasten the edges of the aluminum to the wood with a trim piece, 0.04 in a mild temper (T4 or T6) would be best. Thinner aluminum will wrinkle in the sunshine (thermal expansion is greater than the wood it's attached to, so a loose inner section that is trapped around the edges will try to expand...can't...and will buckle at a certain stress level....which can be related to a temperature).
Can you tell I do this for a living?
Dirk