You could reinforce an existing kick shield with one or more pieces of PVC. Depending on how your shields are stuffed, you could unzip it and shove the pvc in diagonally through the padding or on the held side of the pad, as opposed to the striking side. A single stiffener would go from one corner to the other and two stiffeners would be in an X-shape.
The idea here is that it would spread the load of the impact. The reason it should not be on the striking side is to protect the stick from the stiffeners and vice-versa. Putting them inside the pad allows you to use it normally without it interfering with the users normal grip. An additional benefit would be that the shield would be less prone to flexing and wearing out over time with the downsides being that the extra pressure on the inside may cause the seams to bust more quickly, as well as the difficulty of install.
Some relatively small diameter, maybe 5/16" or 3/8", should be flexible enough to form to the original contour of the shield, if it has one, while still providing structural support. Definitely feel that out at the hardware store, taking the length of your shield into consideration.