Here we go again. This post has been inspired by Fester's article here. Arctic Circle thing indeed. Don't get me wrong, I like his articles. I just don't agree with his (or to be fair, the seemingly most common) interpretation of this rule.
Let's get to it, then. The Kustom Force Field.
"A Kustom Force Field gives all units within 6" of the Mek a cover save of 5+. Vehicles within 6" are treated as being obscured targets." (Codex: Orks, p. 34)
So, vehicles are obscured. Let's see what that means, shall we?
"If the target is obscured and takes a glancing or penetrating hit, it may take a cover save against it, exactly like a normal model would do against a wound (for example, a save of 5+ for a hedge, 4+ for a building, 3+ for a fortification, and so on).
[. . .]
If a special rule or a piece of wargear confers to a vehicle the ability of being obscured even if it is in the open, this is a 4+ save, unless specified otherwise in the Codex." (Big Black Book, p. 62).
So, we need to check if the special rule 'specifies otherwise'. Back to the Codex we go. What have we here? "[A]ll units within 6" of the Mek [get] a cover save of 5+". Are vehicles treated as units?
"A unit will usually consist of several models that fght as a group, but it can also be a single, very large or powerful model, such as a battle tank [. . .]" (BBB, p. 3)
There's further evidence on page 5, if you want it. Vehicles, whether alone or in group, count as a unit.
So, what's my point, then? The Kustom Force Field (or KFF) gives all units within range a 5+ cover save. It makes vehicles obscured, which only tells us we need to apply this rule to vehicles. Targets that are obscured because of a piece of wargear have a 4+ save only if the wargear doesn't state differently (or if it's also obscured for some other reason, obviously), which the KFF does. It confers a 5+ save, and it doesn't matter if you're a Grot or a Battle Wagon.
Following Fester's argument, I'm a Swede, and obviously subject to the Arctic Circle thing, but hey, I feel I make a cogent argument.
So that's my take on Kustom Force Fields. Ta for now.