One of the easiest ways to build up a mechanical frame is to use Aluminum Extrusions, which look something like this:
This type of aluminum tubing is also known as T-Slotted Extrusion, for the name of the channels on the sides where you are able to attach the nuts and bolts of your project. The way it works is that you cut the pieces to whatever length you need, then put in specially sized nuts, called T-nuts, into the channels, and then bolt on whatever other components that you need.
For the balancing scooter, I am choosing to use Misumi 6-series Aluminum Extrusion. It comes with a 30x30mm profile, and lets you use metric sized fasteners up to size M6. Misumi is also great because they will cut the extrusion for you as part of your order.
I am going to be using 3D printing to create the custom mounting brackets for attaching the motors, handlebars, and batteries, and other components.
Here is the OpenSCAD model of the basic frame: