POLYURETHANE might be worth looking into. I dont know if tis reef safe, but I know it is used alot on outdoor projects in the construction industry to glue rocks into place for ponds. I posted the question in the new members area hoping to get an answer myself. I would like to use POLYURETHANE in one of my tanks to glue rocks together and make a bridge.
For those who don't know what it is...it comes in a tube similar to silicone or kalking, and it spreads like peanut butter, hardens in a couple days like steel.
I have repaired broken drills and other plastic cased tools, and it is as hard as any substance in my shop.