So what's in that corner of the PCB? If its 'dead' space, or you haven't had the boards manufactured yet, cutting back the outline for clearance would be an option.
Otherwise, as I said earlier, if the baseplate has holes and the transformer is mounted with bolts, I'd go for some aluminum flat bar stock of a similar width to the transformer core. Cutting a length and drilling two holes is probably going to be cheaper than making a bent steel bracket, and assembly will certainly be easier with only one extra part rather than three for the bent bracket and standoffs. The only exceptions would be if the standoffs are going to be riveted or swaged into the case, (but you said you didn't want to rework the case baseplate), or if the existing baseplate has studs for the transformer, so internally threaded spacers are the only viable option - you've then got to decide between your spacer bracket, and a strip of aluminum bar slightly thicker than the spacers, drilled with clearance holes to drop over the spacers.