I did something similar but instead of cutting PVC in half, I just formed the concrete with a trough built
In the back. I had some slope running from front to back of pad, formed it with a 2x4” laying flat across the back and inset 4” to keep concrete from cracking on the very back. The PVC will work fine, but it will be easier just to form the pad like you want it ahead of time while you are pouring