best stuff to use is fishing line, virtually invisible, cheap and you can remove it once the plant has held
People have been known to use rubber bands to hold rhizome based plants. Often these rot and snap in the tank, but by the time that happens the plant is well fixed.
You can wedge them into nooks and crannies which will hold them on.
People have even stapled rhizomes to wood, but you got to make sure the staples are big and wont crush or pierce the rhizomes! also, getting them out is a pain!! (not a recommended method!)