Some pond stores around here sell big chunks of cork bark, which has a natural look and a rugged texture. And - it floats! They drill holes in the bark... you can get a drill bit that is flat and that drills 3" holes, or a bit for boring out doorknob holes. Put your plant in it's plastic pot in the hole, or just remove the pot and stick the roots, dirt and all, in the hole.
Just don't put it where you have fish, because they will eat the roots and your plant will not thrive.