No species in genus Lucanus requires rotten log or mushroom grown logs at all. Don't worry. You will be fine with just substrate/flake soil. When you set up a container for a pair or a mated female, pour in half of substrate into the container and press it down to harden it, and pour the rest of substrate over without any press. That makes a boundary of soft to hard, and that's where the female likes to lay eggs because females want to lay eggs on safe area where eggs can be secured and safely stored. Female also hardens substrate around the eggs, so pressing substrate will promote females to lay more and more eggs in the container.