I’d just go onto Ebay and do a search for the right sized, or oversized so I could cut it down, angle in steel or stainless steel. Screwfix have a very limited range of metal forms and sheet.
Or even search for a set of the correct rack ears for the mixer. You never know, they may be available or someone somewhere has a set they want to sell or part with.
As for strength, much depends on the design of the ears, the angle of the rack and the total weight to be supported. Mixers and interfaces generally have quite a weight of cable and plugs hanging off them as well as their own weight. 2mm alluminium alloy angle can hold a surprising amount of load, especially if there’s not much overhang and it has plenty of fasteners holding it down to the rack.