If the handle area is going to bend, even just a little, you can make your handle riser out of multiple layers of thin wood strips. This seems to take some bending but probably not much. Another option is to make the handle out of shoe leather. It will flex with the bow. I use shoe leather to fill out the back of bulbous handles and have used it for tip overlays. For the overlays I saturate the leather with super glue and sand smooth.