i'm no expert but foliage is deffinitely a good idea, it increases the surface area mantids can be perched on, meaning they dont all have to crowd on the lid or the tank sides, usually meaning they wont come into quite as much contact with each other and get a bit more space to themselves, with the aim of reducing cannibalism. this could be twigs or branches or whatnot, or even just crumpled tissue i find works well, and will help with retaining moisture and keeping humidity at a good level.