I would love to hang flower boxes from the railings on my front porch; however, I can't seem to find any hardware that would be compatible with the shape/size of my railing. The railings are made out of that plastic/PVC material that is used in new houses, and the top railing is not cylindrical, nor is it rectangular. It has beveled edges, and it is more the shape you would see for an indoor banister. At its widest point, it is three inches thick.

Can anyone recommend a product, store, or DIY method that would allow me to hang window/flower boxes from the railings on my porch?