Do I just click the + in the Den to then move the CUB (from where ever it ended up) into the DEN?

Yes.

Is there a fee efficient way to harvest CUB from the DEC/BUSD farm and move it into the CUB den?

Not much that can be done about the fees. There are a number of moves that have to be made so you have to accumulate enough before harvesting and moving it around.

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