I got the idea when I pondered what to do with a bunch of cheap 3D cinema glasses I have.
Some background information:
One might think that these glasses are not suitable for this kind of use as they are circular polarized, but there's a trick for this. The quarter wave plate has to be on the correct side, otherwise you will only get linear polarization.
You should definitely play around with some of these before you mount them, there's a bunch of strange effects to discover.
Step 1: Stuff you need
A transparen tape looks best IMHO but you can use any old kind as long as it doesn't come off by itself.
I used a scalpel, a x-acto or any kind of sharp knife should do just fine.
For the light diffusor you can use plain writing paper.