There are caliper conversion kits for big front brakes but I doubt there will be for one for your beemer.
Best bet is to source the material yourself, cut it to size as much as possible and then get a machine shop to finish it off. That will be the cheapest way to do it unless another bike kit can be modified.
Buying a 12mm thick section of alloy plate wouldn’t be cheap so maybe even ask a machining company if they have the material already as this will reduce their cost to you (if they don’t have to buy in a plate specially).