Adjusting the big slide per above will do nothing to solve the gap as seen from underneath looking up as the sides are all snug against the side of the rig, it just can't be closed any tighter. Just something in the way they build them. I have used the above to adjust my two front slides though to make them fit a bit tighter when open and or closed but they were fairly good right from the factory.
The gap to me looks like one of those trick paintings that I can never get my brain around. Can't understand how I can have it fit tight on the sides and top and have a gap that gets wider from front to back underneath.