Typically the pohaku used to retain heat in your imu are smooth lava rocks taken from fresh water streams. They should not be any larger than an american football. A friend told me the story of a few guys who stole river rocks from Maui park land for imus. That crime was made possible by the ill fated Superferry.
I suggest scavenging for most dense and smooth lava rocks you can find. That is what we did along the Puako coast. We did not have the time or energy to find proper pohaku in the streams of Waimea.
Pile your pohaku rocks and kiawe into one side of the pit and set it ablaze. Once the fire dies down to hot burning embers distribute the rocks and wood more evenly along the bottom. Add more rocks as the fire settles down to create a base for heat.