Quote Originally Posted by Diapositivo View Post
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.