I tried one of these on the rear brake with a similar set up to the front but it felt a bit spongey and was pretty dificult to set up so discarded the idea I wouldnt have been able to use the seat clamp hanger as my frame has a non standard seat clamp but it might work for you! I dont really have the time or access to the tools to make anything smaller and just wanted to put together something that would work was simple and looked ok that I and others could source from existing parts that are readily available, this forum has given me a chance to display my ideas and steal a few ideas from others keep us informed on your progress!
M@TTY