Unless there are some X-ray film holders in the 20x24 size, I don't think 20x24 holders were ever made on a regular basis. It would all be custom work, and it is much easier to work with wood for small quantities. For the Lisco/Fidelity style it would require molds for the plastic resins, and these molds, even years ago, would have cost thousands and thousands of dollars for only a limited return on the investment. 20x24 was NEVER more than a niche format for film, if that. I think 11x14 was the largest "standard" size for which universal film holders were made and would interchange with each other. Calumet Photographic owns the company that produced Fidelity and Lisco film holders, and they would be the place to inquire if there were ever any 20x24 plastic (style) holders ever made by those companies. If you are concerned about the weight of the film holder, I think the plastic (type) would be heavier than a well-designed wood holder. I have many 5x7 wood holders that are considerably lighter than the plastic ones.