Hard to fill screw holes from the outside. because you immediately trap air inside, and can't force the filler in. Go to the Dollar Store and buy a handful of turkey injectors, fill with whatever you are using, and fill the holes from the bottom to the top, backing the heedle out as you fill. Then throw it away. That’s why you bought a handful.
If using a thin filler, like 2 part epoxy, put a piece of masking tape over the hole, mix it right in the syringe, and poke a hole at the top of the screw hole, thru the tape. Have another piece ready to cover it when you remove the needle