Ah, it came back to my memory another "dirty" (literally) trick in use in the old times, which consists in dirtying an UV filter with a layer of vaseline. The advantage of this solution is that you can control the amount of diffusion. I suppose for your purposes you could try to spread a very thin layer of vaseline.
Not the most practical but very cheap, and validated by a long tradition.
Never tried this personally.
Great minds think alike. I was tyoping same as you were typing. I've never tried it myself either. That stuff is goo-ey and yucky to handle.