How to add a tool from a STL file

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Hello,

I'm new at ROS (kinetic 1.12.17, Ubuntu 16.04 LTS) and I want to know how can I add a tool to a robot, having the tool in STL format.

I'm working with the Robotnik's RB-Kairos robot with the UR10 arm and I am lost with all the URDF files that are in the packages (you can see my workspace at https://github.com/DavidRedrejo/test_ws/tree/main)

I want to add the tool.stl (found in my main branch) to the robot xacro (access here) but I don't know where and how I have to add the tool in the robot's xacro file.

The tool is suposed to be a "static holder" for a real tool, but I think that it is not necesary to represent that tool, just the "holder piece".

Hope somebody could help me. Many thanks for your time!

EDIT: I have made an URDF file for the tool, it looks like this:

<?xml version="1.0"?>
<robot name="spray_tool" xmlns:xacro="http://www.ros.org/wiki/xacro">

    <xacro:macro name="spray_tool" params="prefix parent *origin ">

        <joint name="${prefix}_base_joint" type="fixed">
            <origin xyz="0 0 0" rpy="0 0 0"/>
            <parent link="${parent}"/>
            <child link="${prefix}_base_link"/>
        </joint>

        <!-- BASE LINK -->
        <link name="${prefix}_base_link">
            <inertial>
                <mass value="0.25" />
                <origin xyz="0 0 0" />
                <inertia  ixx="1.0" ixy="0.0"  ixz="0.0"  iyy="1.0"  iyz="0.0"  izz="1.0" />
            </inertial>

            <visual>
                <origin xyz="0.0 0.0 0.0" rpy="0.0 0.0 0.0" />
                <geometry>
                    <mesh filename="package://spray_tool_description/meshes/tool.stl" scale="10 10 10"/>
                </geometry>
                <!-- <material name="grey">
                    <color rgba="0.5 0.5 0.5 1"/>
                </material> -->
            </visual>

            <collision>
                <origin xyz="0.0 0 0" rpy="0 0 0" />
                <geometry>
                <!--box size="0.146 0.05 0.0735"/-->
                    <mesh filename="package://spray_tool_description/meshes/tool.stl" scale="10 10 10"/>
                </geometry>
            </collision>
        </link>


    </xacro:macro>
</robot>

And in the rbkairos.urdf.xacro I also added the include line at the top and this other line.

When I run the simulation, there is no tool, but the link rbkairos_spray_tool_base_link is created, as seen in this frame. (Sorry, I don't have points to post pics here!)

So, the frame is correct but there is no visual display? Or any file is not properly done?