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You did mention that lighting gel before, but I still do not
understand what
is that material and why musical instrument repair shops would have it.

I presume, the material is a gel with dye, deposited on transparent

something to be used to make custom photographic filters. The film must

very thin, to avoid optical distortions due wavy surface caused
by mounting.
In that case, how you would make a 4 x 6" screen without film

Please, explain the construction.

Lighting gel is the stuff that is used in front of stage lighting in
threatres and film sets. It is available in very many colours, in large
sheets, and is heat resisting. It is thinner than the plastic sheet used

front of Tek scopes for sure - but it would certainly be better than
nothing.

Something like this looks
about right for the standard blue 7000 series 'scope filter.

Craig