That looks terrific! If you want to fill that gap, you can cut some chicken wire to stuff in there as backing, mold it, then you get some of that Black foam they sell that goes around an AC Window unit at home (Lowes Home Depot) Then you get a Black truck mudflap, slice it to size slide it around (It bends easy) get it to fit like you want...... and use rubber cement to secure the plastic mudflap to the sponge. You might be able to get a screw in it somewhere or use the plastic fasteners that push in a hole also.
But it looks good as is. Nice fit for BL. Nice looking Rig. MC