I've just returned home from Washington to find an 8cm split in one of our rigid suitcases, presumably where a sharp-edged case was pushed against it in the plane or during handling. The case is made of some sort of vacuum-formed Polypropylene, about 1mm thick. It's not worth claiming on insurance: it's five years old, scuffed, and the hassle and no-claims issues make it a non-starter. However, it's worth spending twenty quid to keep it going.
I've reinforced laptop lids and other flat surfaces in the past using carbon-fibre sheet and Araldite, but this is a slightly curved surface and the case is slightly flexible. I think just running Araldite into the crack will fail, as the mating surface would be just the thickness of the plastic and the slight flexing in use would rapidly crack the adhesive.
So I'm guessing that if I bought some Kevlar braid, soaked it in epoxy resin and laid it along the crack on the inside of the case, then that would be reasonably strong?