How can I improve this gunner's defenses?

[Snafu_RW]

I believe multiple layers of sandbags, while not adding to Cover Bonus, does add to Obstruction Chance since there are more "buildings" the projectile has to pass to get to the target. Though, I admit I have not tested this under controlled conditions.

To add to which, multiple obstructive layers (sandbags, poor ground, some plants etc) spaced 1 tile apart reduce hostile movement by a fair bit, potentially allowing more shots against melee hostiles.. thus reducing turret vulnerability to them

If you're utilising this method /don't/ place sandbags directly next to each other: once the first 'bag is climbed the hostile can hop directly to the next 'bag with no penalty (makes little sense, but..)