With rhino beetles, you can pose and pin (without penetrating) your specimen, and allow them to either dry in a container with mothballs, or naturally in a garage or something (it stinks something fierce) if you live in a fairly arid place. I would imagine you can do the same with mantids. Once they're thoroughly dried the risk to the resin is minimal.
With beetles it's sometimes beneficial to slit open the end of the abdomen and not necessarily scoop out the insides, which will shrink as it desiccates anyway, but kind of just use a toothpick to push them about and make a void. This is mainly so you can pour a bit of resin into the void to help the specimen not float as you pour your encapsulating resin.
Either way I'd highly recommend you get some practice in with specimens you don't care so much about.
