I have seen 2 different ways to deal with the gap -
Simplest is just 2 "U" shaped pieces, rail head width inside and
2X gap distance long.(or there abouts) Just laid on when needed.
The other is just flat stock bolted to side of rail on inside end - so it swivels up,
The other end has "U" that lays over the outside rail. Like an "L" with the lower
leg folded back