Inspection Cover for Fiberglass

I am working on finishing my door interiors and need to find a way to make a good inspection cover so I can access the working end of the exterior lock. I previously cut my hole and added some flanges to the inside of the door to support the panel. I cut a piece of fiberglass scrap that was slightly undersized from the hole and then taped it and the surrounding opening and pressed in some West 407 lightweight filler. I let it flash and popped it off. It helped contour the door to the hole a little better, but it's still not a good tight match up to the hole.

So I'm wondering if there is a better way to go about making inspection panels for something like this or if this is as good as it gets and I should just look at doing some upholstery or other process over the area.