Aluminum comes in different degrees of hardness and mixtures of metals!
What you have in your hand is NOT PURE aluminum PURE aluminum stays soft and malliable.
The 3 TEMPERS of aluminum = 0,T-3,T-6
The 3 metals mixtures I've worked with for over 40 years are = 2024,6061,7075
The lower the numbers, the softer the metal,same goes for tempers.
To make an IDEAL bracket, you can use 2024 T-3 .063 or .071 and bend it with your box break and remember the thicker the metal the larger the bend RADIUS has to be or it will crack. Probably be easier for you to use some ANGLE.
My experience come from AVIATION and the FIRST aircraft I worked on was a F9 Banshee ( LOOK IT UP)
Xnavylfr(CHUCK)