mujoco

<mujoco  model="collide box-shaped meshes">
  <option timestep="1e-3" gravity="-1 1 -10" solver="CG" iterations="10">
    <flag multiccd="enable"/>
  </option>

  <visual>
    <scale forcewidth="0.02" contactwidth="0.05" contactheight="0.05"/>
    <map force="0.01"/>
  </visual>

  <asset>
    <mesh name="box" scale=".025 .025 .025"
      vertex="-1 -1 -1    1 -1 -1    1  1 -1    1  1  1    1 -1  1   -1  1 -1   -1  1  1   -1 -1  1"/>
    <mesh name="wall" scale=".5 .5 .1"
      vertex="-1 -1 -1    1 -1 -1    1  1 -1    1  1  1    1 -1  1   -1  1 -1   -1  1  1   -1 -1  1"/>
  </asset>

  <default>
    <geom condim="1" type="mesh" solref="2e-3 1"/>
    <default class="free">
      <geom mesh="box"/>
    </default>
    <default class="wall">
      <geom mesh="wall" rgba=".5 .5 .8 1"/>
    </default>
  </default>

  <worldbody>
    <light pos="1 -1 1" dir="-1 1 -1"/>
    <geom class="wall"/>
    <geom class="wall" pos="-.6 0 .4" euler="0 90 0"/>
    <geom class="wall" pos="-.2 .6 .4" euler="90 0 0"/>

    <replicate count="5" offset=".06 0 0">
      <replicate count="5" offset="0 .06 0">
        <replicate count="4" offset="0 0 .06">
          <body pos="-.1 -.2 0.15">
            <freejoint/>
            <geom class="free"/>
          </body>
        </replicate>
      </replicate>
    </replicate>
  </worldbody>
</mujoco>