Well, https://tpt.io/:22932 (it's actually better to ask things like this on the forum)
Uh, brainfart. Ignore the comment below, that's actually a half-opaque mirror. I know there's a way to do it, I just don't remember how. Hold on.
BMTL together with something opauqe (say, DMND) acts as a one-way mirror, see top-right quadrant of id:2316631