Creating a toolbox component with a single variable

Hello, I want to create a component (and have it in toolbox) which is nothing more than an aluminum profile. The only value that changes on this component is the extrusion length.

I managed to create a part and then with the publisher configurator to create this variable. Each time I insert the part, I enter the length and it works, except that with each new insertion in the assembly it opens me, then closes the excel sheet of the family list. It's painful.

If I do the operation to add a part in toolbox, it works my new component appeared with its particular icon, but when I insert it in my assembly, the length parameter I had previously disappeared. When I go to "configure the toolbox component" I don't have this field to enter the length. If anyone has a lead? 

Thank you