assets/mjcf/brick.xml

<?xml version="1.0" ?>
<mujoco model="brick">
  <asset>
    <mesh name="mesh_brick" file="../obj/brick.obj"/>
  </asset>
  <worldbody>
    <body name="brick">
      <geom name="brick_geom" type="mesh" mesh="mesh_brick"/>
    </body>
  </worldbody>
</mujoco>