You can make your own frag plugs!!! Just get some concrete, aragonite sand, and water to get started. You’ll have to experiment with the concrete to sand ratios, but the more concrete you add, the harder the plug will be. This also means that you will need to cure the plugs before putting them into your aquarium. If you use more sand in the mix, the plugs will be more brittle, but at the very least, cover the top of the plug with sand for a more natural look.