Something like this?
View from inside the box, looking up. The object on the left representing the L notched to be bent and welded into the shape on the right.
I've not used L in this way and I'm not sure about the corner. I've found that L as an edge stiffener is less resistant to deflection than I'd anticipated. I'd be nervous about 3mm steel L over a 40" (100cm) span with 500lbs on top. An 8' (244cm) bookshelf where I tried to stiffen the edge with L steel so I could keep the height of the shelf short resulted in having to hide steel cables between books running 45deg back to the wall in 3 places to get rid of the sag.
View from inside the box, looking up. The object on the left representing the L notched to be bent and welded into the shape on the right.
I've not used L in this way and I'm not sure about the corner. I've found that L as an edge stiffener is less resistant to deflection than I'd anticipated. I'd be nervous about 3mm steel L over a 40" (100cm) span with 500lbs on top. An 8' (244cm) bookshelf where I tried to stiffen the edge with L steel so I could keep the height of the shelf short resulted in having to hide steel cables between books running 45deg back to the wall in 3 places to get rid of the sag.