Reasonably priced BIG BRAKE alternative that fits in a 15” wheel

Ok, what type of spindle? Disc or drum? And what did you use to mount it? Because looking at that caliper I see a whole host of mounting issues. This isn’t to be questioning what you built, but overly complicated mounting that the average guy can’t fabricate without a machine shop isn’t helpful in a backyard situation. a $25 GM truck caliper works with mine also with the benefit of a production 12” over the counter rotor. It’s not about fancy but cost effective and simplicity.