Sebaes are a sand dwelling anemone and while they may attach to live rock, if lighting conditions are acceptable in that area, they prefer the sand bed.
Assuming you updated to the 70cube you were contemplating back in March, and got a 6 or 8 bulb T5 fixture you should be fine was placing the anemone on the sand bed.
Choose an area with moderate flow that will allow the tentacles to gently sway. Then dig a small hole and place the anemone foot first in the whole.
To keep the current from knocking the anemone over while it gets used to where it is and hopefully extends it's foot into the sandbed, I would suggest getting one of those plastic fruit baskets to use as a cage. This will keep any fish you have away from the anemone while it establishes itsself.
Also ensure you leave lots of room around this anemone to grow because it will get huge and sebaes do not split.