If you are interested I built a FHM "condo" for my RES/SMB pond to give the FHM something to spaqn on. It was not in a cage but instead staked to the bank of the pond. But it might give you some ideas.
Did it work? Darned if I know. I got the idea from some youtube videos on FHM or Rosie Reds spawning in aquariums. Several people were using PVC pipe as a substrate for them. So I got the idea of making a colony of PVC pipes. I tried to turn the pipes different directions to give the couples the most privacy and minimize competition. The holes drilled in the pipes were for water flow and good aereation (at least my idea of it).
Scroll down the page to where the title includes the words "some FHM substrate. 2/3 the way down the page. The installation of it in the pond is further down that same page.https://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=459159&page=6