Some TLRs allow you to reset the focus of the viewing lens, which has its own helical. I don't know if that's possible with your Brilliant.
A mirror from a Polaroid is very thin, although you probably would need to cut it.
Originally Posted by momus
I'm not sure whether to put this in the Fixed or Broke thread. It all started (doctor) when I thought I'd attempt to get my Voigtlander Brillant TLR to focus easier. I pulled the WLF off and the mirror was awful. Took a bit of lens tissue, gave it a very, very soft wipe, and all the silver came off w/ that one wipe. OK, that didn't work. Tried to cover it w/ tinfoil. Put it back together, couldn't see a thing. Next, I cut down an old 'cord mirror I'd replaced some time ago. Got it in fine after the trim, but now the focus was WAY off. Turns out that the mirror is nearly twice as thick as the original. Pulled it out, again, and flipped the mirror so that the silvered side was now on the bottom. This got the focus closer, but I had to take the WLF off about 10 more times, and keep adding strips of tape to the bottom to get the focus right. It's OK now, but that mirror is not that great, so I will probably have to do this all over again once I find another mirror.