How do I create custom layouts for Panelizer?
Panelizer is based on Panels module, which uses Layout API module in Drupal Core for providing  layout plugins. Please refer to the relevant documentation:

https://www.drupal.org/docs/drupal-apis/layout-api/how-to-register-layouts is written with newer Layout Builder in mind, but basics should be the same
https://www.drupal.org/node/2578731 is an older version of the same docs - might be slightly out of date, but also contain something not present in the previous document