bit of spare perspex, left over from your hood project, drill some holes through it, so the water goes through it, couple of rubber suction cups to keep it in place
2
you know the fine mesh wrapper oranges and onions come in.
depending on size knit a couple of them together with your fishing line
tie some rubber suction cups on for the sides
put the base under the gravel
you could also hold it on there with some old algae cleaner magnets ( if you had any ?