the "tilted glass" technique wont work for a mantis. they will be able to swim out. the glass will only work for crabs.
i had to deal with one about the same size in my tank. here's how i got mine out.
take an empty 2 liter bottle and cut the top off... just below the point where the rounded top meets the straight side wall. make sure you remove the cap before procedeing any further. apply a bead of silicone around the inside of the rim of the bottom part of the bottle. take that top section and turn it upside down and insert it into the bottom part of the bottle. basically, it will look like a funnel inside the bottom half of the bottle. let the silicone dry for 24 hours. i cut a couple tiny slits in the bottom of the bottle to allow air to escape (not big enough for the mantis to fit through).
to set the trap, place some small pieces of raw shrimp in the bottle, and put the bottle in your tank. this is where the small slits really help get the air bubbles out. place the bottle opening right by the location of the mantis. make sure the bottle is lying flat on the sand, or tilted slightly upward. you do not want the mouth of the bottle pointing downward. i used a rock to weigh down the bottle, because it is has a tendency to move or float away.
the mantis will eventually smell the shrimp and will enter the bottle. but, it will be unable to climb out of the bottle because the exit is located in the middle of the water column. you may have to try this method a few times, because if you leave the bottle in there with a rotting shrimp, you will get some bad effects on your water chemistry.
oh, you may want to cut a trap door in the side, and then seal it with tape. that way, you can get the little bugger out once you catch 'em. also, if you do not want to silicone the top inside the bottom, you can use clear packing tape cover the seam between the parts.
FYI, it took me about three nights of trying before i caught mine.
DIY stand and Hood,
Coralife Lunar Aqualight (4x65W),
60 LB Hirocks Base Rock, 40 LB LR 50/50 Keys/Gulf
150 LB Playsand (~3-4 in DSB)
2 x Blood Red Fire Shrimp, 2 x Cleaner Shrimp
2 x Black and White Percula Clown, 1 x Orange Spotted Blenny, 1 x Red Stripe High Fin Goby