Greetings once again!
I would like give several reasons why under-cage heating may not be as benificial.
For example- if you were to use an adhesive heating mat on the side of an enclosure, it would provide a heat gradient. This would allow the mantis to choose the best temperature, while being able to regulate it based on his or her preferences. The heat would spread evenly, farther out would be colder, while closer would be hotter. This would be the easiest thing to do. They are also fairly cheap from your local pet store. See what he or she likes- if they move closer to the heat, they obviously like it. Vice versa. Move it accordingly to their preferences.
If you decided to use the heat lamp approach, here is what I would suggest. Make sure the lamp has a low wattage. I can't say for exact what wattage you would need, your going to have to experiment a bit. Do a bit of reaserch of the species you intend on housing's prefered temperature range. Put the lamp over one side (a couple inches away to lower risk of the fabric igniting and/ or your mantis overheating). Once again, move accordingly.
I hoped this helped and feel free to ask any questions you have!
