I don't really have personal experience, but some basic facts...
Definitely a sandy substrate, maybe some rougher coarser substrate would be great..
The ciliata
mantis prey on fish, as you know...so watch out what you put in there, I have heard of people feeding their spearer mantis feeder goldfish and the like. Sometimes they don't want to accept dead food and you have to encourage them onto it, probably with some pieces of silverside or something similar.
They dig burrows, so maybe put some extra substrate under your rock, that way there is room to dig?
Hope that helps...