I have not seen any heron at my pond but have seen them at a nearby field pond so I am certain they are around.
Earlier I mentioned " and could be stacked when placed off to the side"...well, if done carefully, they could be sized such that, when turned upside down, they could fit inside each other in an ordered manner. Much like stacking pots/pans in the kitchen cabinet. One would be the largest and the next one down in size would need to be only a few inches smaller in either direction and then the next one even smaller. Given that you have an oval pond it might be pretty easy to accommodate that sizing because the shape of the pond changes.
Earlier I mentioned " and could be stacked when placed off to the side"...well, if done carefully, they could be sized such that, when turned upside down, they could fit inside each other in an ordered manner. Much like stacking pots/pans in the kitchen cabinet. One would be the largest and the next one down in size would need to be only a few inches smaller in either direction and then the next one even smaller. Given that you have an oval pond it might be pretty easy to accommodate that sizing because the shape of the pond changes.