Currently I've got mine sandwiched between 4 ply matboard. The idea is that I can touch the mat board when taking them in and out, as opposed to dirtying up the gels themselves.
Can also make a glass "hinged" sandwich with the gel in the middle and insert that in the holder. I have several permanent ones with tape all around for the ones I used the most and then a couple of blank where I could insert gels as needed.
If you have the heat absorbing glass (DURST LACALO) installed on your 138, plexiglas should be ok, no reason the plexiglas cannot do as fine as the gelatine filter itself. Your carboard cut probably too much light.
Believe it or not, the mat board only gobbles up 1/3 of a stop of light - I've tested with my Stouffer step wedge. They were only meant to be a temporary solution, but that was 6 years ago.
If I can get some glass cut to size, that's definitely the way I'd like to do it.
I use gels only for certain masking applications in contact frames where I need a very specialized color, and never for projected images.
They're fragile and expensive, and for anything related to black and white printing on VC paper you can buy appropriate glass filters. All you
really need is a deep blue and deep green. But I have colorheads anyway. If you do use glass filters beneath the lens, all it takes is a simple
thread adapter, just like on a camera lens. But my 138 has an adjustable swing-away filter holder built in below the lens turret.
It is a luxury, at least. I use mine for both threaded glass filters and well as for a threaded gel filter holder. No different than putting filters
over a camera lens. Sometimes older 138 units are missing these holders. They come up for sale from time to time. Or you could just improvise something like this if you can spot the original attachment point. The filter holder should swing into position directly below the lens
axis, then be adjustable in pin length for various length enlarging lenses. I don't know the official part number.
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