i thinking a resin or glue would harden the outer polyester type fabric of the case but i‘m not certain
My problem with hardening a cloth case would be holding the shape while the material hardened without getting the guitar wet from the glue's water or the resin's solvent. And cloth cases tend to fit so closely that as you are putting the guitar in, they bend and change shape to fit and if the outer surface is stiff, they are less likely to do that. I think you would be better off either shopping around for choices on hard shell cases, perhaps for one that is lighter than the most common ones or looking to make a shell of thin stiff plastic material that your cloth case would slide inside to pad the guitar.
