i admit that i have never used biochar but in terms of producing it, what about a butane gas cylinder, either the ones used for domestic cooking or smaller ones used for camping. the steel is definitely strong enough to last. if its cut into a large piece maybe 80% of the total volume, the remaining 20% would be used as a top. The larger lower part would have its top bent in to allow the other piece to sit on top in the form of a cap or lid.
