Body working odd shapes. Something like the inside of these fender aprons can be a challenge. To make life easier, I first established just the edge that goes around that pocket. Once I was happy with the shape of that, I spread the filler on the inside of it , and used the spring steel to drag trough it. This creates almost the exact shape needed, using the corners you already dialed in as the guide. And the other advantage, is because of how the spring steel bends, it creates a nice natural looking shape. Try it out next you you have something like that.