I think we're talking about two different things. The traditional drop-bolt
replaces the centrebolt of the brake with an offset one to lower the caliper arms (works well when you only want a modest-drop, like 10mm):
If you need more drop than that, then there's sufficient room to create a drop-bracket
that lets you mount the same brake lower. In these cases, you MUST
make them double-sided to deal with the single-shear cantilever forces:
Due to the wider bracing, you can use thin material, 3mm steel or 5mm aluminium. If you want to make a single-sided one, I'd go with no less than 10mm aluminium.