Reef balls are great, however they are large and very heavy, requiring heavy machinery to place them.
Artificial reefs similar to the above could be made with concrete in concave holes on a beach near to where they will be placed. This could be an activity for both children and adults at holiday resorts located near the sea, to expand existing reefs or make artificial reefs in more accessible places.
I think that three “mouse doors” at the base, and one hole at the top would be better. If they are about 70 cm to one metre (one yard or less) in diameter, and about 10 cm (four inches) thick, they could be carefully placed on a tarpaulin, before sliding them down the beach, and once in the water, one or perhaps two strong people could carry them out as deep as they can at extra low tide. The reefs would then be within easy snorkelling distance and depth for tourists.
I think that such an activity could increase tourist numbers and satisfaction for tourists visiting the Caribbean, South-east Asia, Pacific islands, the Red Sea, East Africa, Indian Ocean islands, etc.
Resort owners/operators may also get repeat custom, because some tourists may return to see the progress of the reefs they have made.
No-fines concrete could be useful for making artificial reefs. The aggregate could possibly be coral rubble, or even construction rubble, however a dome or igloo shaped reef made entirely of no-fines concrete would probably not be very strong and is likely to crack or break. A possible compromise would be to use normal concrete with an outer skin of no fines concrete, because the rough surface with cavities is likely to increase the recruitment of corals, sponges, seaweeds etc, and provide homes for small creatures.