You can build one out of a 20oz coke bottle. Cut the bottle off horizontally just below the upper curve of the bottle. You'll be left with the bottom about the size of a coke 12oz can, and a curved plastic piece with threaded top that looks like a funnel.
Turn the upper half upside down and shove it into the bottom half, then secure the two halves together. I use wire ties.
Stick a piece of shrimp or something in the bottle along with some LR rubble to weight it down and drop it in. I've had varying success, but have never tried to catch a Mantis with it. I just remember when I built the trap that the tutorial was for a Mantis trap. Critters crawl in through the inverted curved top piece and get trapped in the bottom.
If you know where it's hiding, it may be easier to pull the rock. Extract some tank water into a bucket with a top opening far bigger than the rock. Pull the rock out, hold it over the bucket, and squirt fresh water in to the hole where the Mantis is hiding with a baster. He'll come right out and fall into the bucket. Don't kill the shrimp unless you have a serious revenge complex, as there are usually guys on this message board who want them.
Supposedly hot water and kalkwasser work better, but they'll almost certainly kill the shrimp. I've also read to submerge the rock in fresh water, but I'll bet that seriously harms the life on the rock.
...you could try grabbing him with a tongs... ever seen Karate Kid? "Man catch fly with chopsticks, do almost anything."
