The best additive is sawdust, because morel mycelium likes that texture. But there needs to be heterogeneity to create openness and let oxygen in. Sawdust is usually free at lumber yards. It collects under the saws. Chopped corn cobs are very good.
Shredded aspen is very good.
These mediums alone will not have enough water holding capactiy. The best additive for that is black peat. Sphagnum peat is not good, because it is too acidic. Potting soil could be used. It would be improved by adding sand.