If you want to start creating your own custom renderings (completely new one or by cloning existing one) I would suggest creating a new feature. SXA has scripts for this, take a look:

Then you can simply extend your Site with this feature, there is also a script for this:

Ok, you have your new feature (the place where you will be creating your own renderings) so you can easily create new toolbox section. The tricky thing is that it will be created not from the name of an item under /sitecore/content/Tenant/Site/Presentation/Available Renderings
but the name will be taken from an item under which you've stored your rendering. So:
- Create a rendering:

- Add it to available renderings (under your site):

- Done