
I mostly use those gradients located at the lower right hand corner of the jpeg image and stack 2-3 sheets together, per Litonotus' advice.
For those of you located in USA, you can easily and cheaply print the image on a letter-sized clear film at OfficeDepot (cost is around $1.5 per page - I suggest printing at least 6 sheets).
My experience with GUF is that, with high NA (>= 0.85), GUF has to block out any un-interfered light. Otherwise image would end up looking more like oblique than DIC (less blocking, less contrast). On my scopes, bigger and wider GUF produces a more DIC look, as more lens aperture is blocked/interfered.
Certain condenser with circular filter tray and bottom diffuser lens, such as that of LOMO Biolam or Zeiss Jena, would work better with bigger/wider GUF by having filter tray and diffuser lens removed and GUF attached via mounting putty (such that more aperture is blocked/interfered).
And here is two actual GUFs that I made using plain microscope slides, gradients and tapes. I use the narrower version for rectangular condenser filter tray and wider version for circular tray.
