Sugar is a desiccant, meaning it will absorb moisture from the air. So putting it on top bars will have it absorb any heavy moisture that might be in the air inside the hive. As the sugar gets wet from absorbing moisture, the bees suck it out as syrup. So, putting wet snow ball consistency sugar on the top bars actually feeds the bees syrup. Bees don't eat solids, they crush up solids with their mandibles, and suck food as liquid down through their hollow straw like tongue.
Using follower board is fine in my opinion, it will only take one to work. I would put a frame of honey against the most warm side of the hive ( a side facing either south or west) then the brood frames, then any other stores frames, then the empty drawn frame or a foundation frame, then the follower board. If the bees are not populous enough to cover 4 frames, then I would not put a foundation frame in yet, it can be moved in later as the bees grow.
For added insulation, I could put a block of styrofoam or insul board in the space outside the follower board.
Using a follower board instead of a nuc box will cause a bit of issue using the top sugar, or mountain camp method as it's known by in this forum, because it'll require a feeding shim under the top lid to give room for the sugar. That means the follower board won't reach up to the top of the space under the lid, but it won't cause much problem other than lack of sealed air flow. I think the bees like to feel crowded and cozy, but they do fine besides what I do with them most times.
I still say a five frame nuc box would be better. Besides, every bee yard should have a complete nuc box setup in the beeyard for any situation that might arise that might make it come in handy. A place to put frames as you are inspecting a hive, a box to make a split with, a box to hold the queen on a frame as you are inspecting and etcetera. Perhaps you'll decide to get one for your yard soon.
I have found that using the sugar snow on top bars, the bees cluster up under the sugar, and keep the sugar dried out from sucking out all the syrup that it creates. I've even seen them store excess syrup from the sugar, excess in that they don't need it at that moment for feeding.
Good luck, it's sounding to me like they'll most likely make it fine and in good shape whatever path you take to secure them.