Isolation is forerunner, but there is a massive flood outbreak that covers almost half the map, I would include that somewhere. And I've never seen any proof that anything but sandbox's towers and the forge grid are forerunner. I think it's sort of a mystery who makes the ancient sand textured structures, I could be wrong. And for snowbound, since you think there are hardly any covenant maps you'll leave it there? Maps don't have feelings, they won't feel left out if they have a unique look, I don't understand what your motives are here..
Sandbox seems to be pretty heavily based on the aesthetic of Sand Trap, which would lead me to label it as Brute in origin. Whilst this is isn't stated anywhere that I know of, it's pretty clear that the Brute premise is true of Sandbox in terms of aesthetic atmosphere if not in direct back story.
Sandtrap was occupied by the Brutes, but it is of Forerunner origin. The same idea would apply to Sandbox. Also the Isolation debate: It is a Forerunner containment for the infection that spread around the area.
Description for Snowbound. I read the description for snowbound and it says "Hostile conditions did not prevent the Covenant from seeking salvage on this buried Forerunner construct". So my conclusion is that it used to be Forerunner but is now Covenant. Although I have a feeling this debate has not ended. Also should I include a Flood group?