How should I go about transplanting the minicube into the rock filled planters?

Simply fill the planter to about two inches from the top, set the unwrapped minicube on the rocks and fill in around the rw to to fill the planter the rest of the way, covering the rw completely.

The small fine grade (almost sand like) grow rocks are great for sprinkling in, on top of and around the edges of the minicube, they help wick solution in and out of the rw better than the regular larger grade grow rocks.