If you are feeding standard hives, and they need feed, I have found that placing the fondant on the inner cover hole, then placing an empty box around the fondant, this is the way to go. Many times, especially when bees are starving, they will be at the top of the top box, as seen when you see bees at the inner cover hole during winter, when they should be further down.
I think that is for two reasons. 1) they have eaten their way to the top and 2) They are at the top trying to conserve energy and using any trapped heat benefits.
So placing the fondant on the inner cover hole is an ideal location.
Fondant also, if the moisture is high, may "leak" or drip in some situations. Having the fondant on the inner cover hole, only allows the bees access to the portion provided by the inner cover hole itself. As they eat into the fondant, this will open up and I have had bees hollow out the entire inside of a 25 pound block with no issues.
I'm sure there are applications where perhaps feeding fondant inside by using frames makes sense. I just happen to feed fondant to light (NOT weak) hives who would otherwise starve without it. And this would be considered emergency feeding, and on top of the inner cover makes more practical sense.