Bubblewrap.
Cut a square, maybe 10cm x 10cmish. Float it on the water surface... I duct tape it to the side so it doesn't float everywhere.
I make two squares, and pop on bubble side down, and the other bubble side up, both next to each other.
From previous nests/spawns, I've noticed the male will start nesting on the bubble side down. If he's one of those fancy nest builders all about size, he often continues to bubble side up.
The bubble wrap looks like a pre made nest to females when the male hasn't started... seems to get them ready, at least that's what I think. Also gives her an indicator of where the nest will be, and being female - I'm sure she wants to know all the details!
For the male it helps him get a start on the nest and hold it together. Often enough if a male sees a nest, he'll continue it and make it his own... although that's not always the case.
I've used bubble wrap as a nesting media for maybe 2yrs, and haven't had a male make his nest elsewhere ever... so really good for those tricky males that hide and make their nest in an awkward spot for photos.