Good news that it has stuck to something, many wont for quite a few weeks or months.
I would likely try the tupperware with 1" or so of sand and one piece of softball+ sized rubble. Move away the sand at the point the rubble and the rock meet, even better if the rock has a cave like opening at that point so the anemone can actually attach to the rock and the tupperware. Back fill a bit more sand around the anemone after it attaches. I find they move in the tank/tupperware far less when they find a nice rock/cleft in the sand.
Maybe get the sand level up to about 2"-3" over the next few weeks.
Alternative is to clear sand away from a similar rocky position in the main sandbed, not sure about your setup, needs for possible relocation of anemone later, etc., etc. I have found tupperware more convenient for me, maybe not the anemone, but me :D
fwiw: PM me on RC for a quicker response.