Help with flask project SQL ERD

I am working on a Flask capstone project where I have a toolbox web app. The user creates an acount and logs in. Once in they can see a list of tools available and create 1 or several toolboxes with their tools in it. They can add or remove tools from their box from the full list of available tools. This diagram shows what i have so far, but I’m getting stuck on how to build the relationships? One user can have many toolboxes. Each toolbox can contain many tools. This is what I have, but I feel like I’m missing something? Any help greatly appreciated (especially when doing the relationships in python models)