I bought this Hygger lamp (https://mcmerwe.co.za/shop/hygger-submersible-led/) never intending to submerge it. I designed this mount for the light to hook onto the wall of the aquarium. I am attaching the source files as well in case you need to modify it for a snugger fit. This is made for a 5mm sidewall.
You push the light through the opening on the end. Rotate it, and snap it into the mount. Check the images if you are confused. It should be a fairly snug fit.
The author marked this model as their own original creation.