How to maximize inductance of a homemade coil?
More turns increases inductance and packing the turns closer together (so that they all share each other's magnetic field) maximizes inductance for a given number of turns. Using a ferromagnetic core helps all the turns of the wire share each other's magnetic flux. If the ferromagnetic core becomes a full loop - then the inductance increases dramatically as the "gap" reduces to zero.

Ultimately, on an ungapped core inductance is proportional to turns-squared. It's also proportional to the magnetic permeability of the material.

If the ferromagnetic core is conductive this can dramatically reduce inductance at AC frequencies because the core acts as a shorted turn - this is why power transformers have cores made of insulated laminates of iron.