ADHybrid,
Acrylic is more shock absobent than glass.
Much would depend on the species of Mantis as well. Mantis with spears for the front apendages wouldn't matter. The clubbed front claws are the smashers.
Most Mantis are relatively small, however a few species can outgrow a 12 gallon tank.
I hope your creative or artistic. Place some rock into the substrate and create a few caves or crevices in the front. Slant them so there not head on with the front glass.
Otherwise your new guest will find a burrow in the back out of sight and you'll never see it.
You can use small pieces of PVC and conceal them with the rockwork.
Good Luck
Ed