I use Smart and Final light weight spakling paste found in most hardware stores for most filling required. The tub it comes in weighs more then the paste. Open it up before buying and make sure it is fresh, should be nice and smooth, a bit moren then tooth paste. Fill in the gap, let set over night, sand then hit it with CA. It hardens just like wood. If you need it thinner then just whip it up with some water. I even use it for filling in dings and the little hanger rash you get on a build before I cover. Good stuff to have on hand in your shop at all times. If it hardes up too much over time just add water.
