It will always fall through the gravel. Gravity is king. You can get a 1/16" silicone,semi-rigid cutting board and cut it down to create a retaining wall of sorts. Remove gravel in an area/corner and fill with sand. You kind saw the piece down through gravel while keeping 2 ends supported to keep it rigid if that makes sense? You want the bow towards gravel. You may have to cut a spacer to hold form while you fill in with sand. Once sand is in it will hold its shape.
I'm not really here... fell in the tank and my phone is just that smart