rigging up a left hand rear brake when non is available

I really want an lhrb for my wr300. Problem is the rekcluse one doesn't work for those tiny rear master cylinders on the two strokes. Is there some kind of universal adapter out there I could buy that would screw into the master cylinder that the lhrb could screw into or mabye a t-line or something? The master cylinders are around $200, I dont want to spend $500 to make this work

Have you looked into a MTB version?

No , I have heard of people doing that but would like to know exactly how they plumbed into the existing master cyclinder

I used a Honda kit by changing my rear master to a Nissin one (stole it off my honda 125, shhh). I picked up a replacement to put back on the honda for $40.

Just had to make up a simple bracket since the bolt spacing is different (wanted it reversible, so no cutting off the stock bracket/welding on a new one)