I've had a pair of the AudioMoth underwater cases for a little while now, tentatively testing them out in our backyard pond. However, very soon I'm keen to deploy a couple of units on a shallow reef to observe the soundscape of a seagrass meadow over extended periods. The potential for this is enormously exciting. For securing each of the units, I've been trying to think of an object which would be heavy enough to keep the unit fixed in its place to the sandy substrate, not get pulled away by the tide and also not interfere with the environment as much as possible. I've thought of maybe using a couple of small dumbells and securing around the handle with a bit of foam to hold the velcro strap in place. I'm curious if anyone else has suggestions for securing their units - any thoughts?