SF structures initially was made it to get SF Crops snaps, but today all SF crops has snaps on any structures, except on ceilings, which by default don't allow this snaps functionality.

So SF ceilings allow this snaps functionality, but the greenhouse effect is the same as the Ark.

And finally, it isn't possible to make the greenhouse effect work in shields. This is an unexposed function that doesn't allow us to change.

There's an workaround that already was reported by some players, that works is if you disable the shield and enable it again, the greenhouse effect will work. But when you leave his field of vision, when you return you will need to do it again.

There is an older magical seeds mod ID#700624475 where the crop plots get the greenhouse benefit irrespective weather you are in a cave or under a shield and with the introduction of the Tek Crop plot in Genesis 2 ... Itd be good to see this mod follow suit esp since a lot of folks build in caves an use forcefields for protection.