The idea of this paper is the application of biomaterials in construction, primarily as thermal insulation materials. It is necessary to prepare biocomposite specimens to test their mechanical characteristics and behavior. This work prepared the specimens for three-point testing bending following the corresponding standards ASTM D790 (three-point bending) and ASTM D7264 (flexural stiffness and strength properties). Firstly, a 3D polymer mold prototype was developed and manufactured. After that, the technology of the biological process of the production of mycelium-based composite materials was carried out according to the appropriate procedure, where three different types of substrates with two types of granulations (fine and coarse) were used. Specimens (or plates) with a reinforced structure (where the moisture percentage was reduced to a minimum) were obtained and prepared for the mechanical characteristics testing.