. "Cardboard holders? Sounds like the Beverly Hillbillies"
You should note, that "hillbillies" from Beverly Hills or Appalachians in general, are some of the most inventive folks this Nation has ever seen, and yes, it is through their 'Need' no to allow poor (monied) circumstances, that they do no sit and wait for the day they may be able to buy new 'things', but rather because they have things to get on with... and do so!
It is also a lot of fun and there is great satisfaction to being inventive, daring and interesting because a person can improvise and hold forth over many of the obstacles to daily life or special need.
Matt board negative holders are kinder to the negative and can be made to fit any film size, up to 4x5 inches, and there is no reason to think that gel holder, like the one I describe, would no serve in light of the lack of optical glass filters, sized for many different lens diameters, or gel filters of 7.5cm - 10 cm.
Just because a product exist, does no mean that you or others must go out and buy it, rather than simply making your own substitute from materials on hand.
Of course some folks live in fear of being seen with such inventions, as people might call their solutions "ghetto", "PO!", "jury rigged" or even "hillbilly" or have their 'social status' lowered in the closed minds of others, by no having the "proper" equipment for their cameras.
If it works as a solution or tool, use it until you can find/afford better, and in the mean time, let distractors have their say, it is no your dignity being insulted, but, their own lack of respect for others being exercised.
IMO.