If you want a product that will fill anything, try a can of Bondo. Not the Spot Putty, but the can of resin used in body repair. I had tips of plywood fins get crunched (“smooshed” is the description that might best describe the injury ...). I needed to form a new, sharp-edged tip, and using regular epoxy would have been harder to use, and have it keep its shape.
The Bondo dries really fast, like in a minute or so it becomes difficult to do any additional forming. But this is maybe it’s best feature. Mold it by hand and it will keep its shape with hardly any wait. And that means no sagging. It seems to bond to anything. My daughter has an old mid century modern laminate sideboard. It had cracks and voids in the laminate. I fixed it with Bondo. It sanded pretty well. We then sanded the whole piece, and she primered, and painted it. Looks like it never was repaired.