Any formation will provide hard cover, I can explain it again more briefly this time.
Each firing MODEL (not the unit as a whole!) must see 50% or more of the target UNIT or else suffer the -2 hard cover penalty.
Yes, there might be a gap right in front of each firing model, but each guy must have unobstructed view of 50% of the entire target unit to avoid the penalty, which I really don't think would happen even with a straight line of skirmishers 10 across in front of the target unit. Each model would see 1/2 inch of the target unit through the gap right in front of him, but he's going to see much less through the gaps that are off at an angle. Get down to eye level and try it out..