I'm waiting on a shipment of three anubias nana plants to come. Should be here by tomorrow. I intend to weigh it down to substrate in the same way that I weigh my nana 'petite' down. With a paper clip.
Just use a vinyl coated paper clip. Cut it so that you're left with a: U shape. Hook it over the rhizome and push down into the substrate. Leave the rhizome exposed, the paperclip shoul dhold it. Then the roots will grow into the substrate, anchor it, then you can remove the clip.