For my rhino beetles I gathered rotten oak wood, baked it at 250 for 20-40 minutes, and mixed it with organic garden soil. Making sure the substrate is moist enough and supplying them with dry cat or dog food for extra protein. I just did this yesterday so I will see how they like it. I am also going to make my own fermented substrate with oak pellets, adding water, organic soil, flour, and yeast (letting it sit for a few months to rot).
Hope this helps. Feel free to follow my blog as I'll be keeping track of all of my ventures.
bug-boy-charles.tumblr.com
Hope this helps!
-Charles