How to properly set JACKAL_URDF_EXTRAS?

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Well you can do in two ways:

1. JACKAL_URDF_EXTRAS with this you have to do the same procedure as you did above. I think you also have to publish the static transform as well from Jackal to your Lidar.

2. Adding accessories to the robot. This thing is fairly simple and worked quite well for me and also rendered everything in Gazebo and handling was quite easy.

Here is a small example:

<!-- front laser -->    
    <xacro:include filename="$(find warthog_description)/urdf/accessories/lasers/front_laser.urdf.xacro" /> 
    <joint name="fl_joint" type="fixed">
      <origin xyz="0.025 0.0275 0.025" rpy="1.5708 -0.7854 0" />
      <parent link="fl_frame"/>
      <child link="f_laser" />
    </joint>
  </xacro:if>

and the front_laser.urdf.xacro look something like this"

<?xml version="1.0"?>
<robot xmlns:xacro="http://wiki.ros.org/xacro"  name="f_laser" >
  <xacro:include filename="hukoyu.urdf.xacro" />
  <xacro:hokuyo_ust-10lx_mount prefix="front" topic="front_laser/scan" location="0.07"/>
</robot>