How large is the tank you are trying to find him in? Is taking out all the rock an option? If you can locate the rock he is in it is much easier (just take the rock out and let it above a bucket of tank water (only a couple gallons will do) and wait) Both I have caught that way it took less than two hours for the shrimp to move to the water, you culd leave some smaller rubble in the bottom for the mantis to get comfortable yet be able to be caught or seen, your option.
If you can not remove the rock, I like the soda bottle trap that a few on thsi site have posted, although I have never used it , the idea is just like a crab trap, should work in any tank. Mantis are very smart and seem to learn from their mistakes, so if you fail to catch the opportunity to snag him, it will likely not fall for the same trick twice.
Just my opinion there are far greater knowledged people on this site as to how to catch these guys.
If you get ready to let him go, remember my PM to you,
Thanks
Dav