dpkg-buildpackage -b -d
cd ..
sudo dpkg -i cosmic-panel_0.1.0_amd64.deb

See the provided configs for the panel and dock in data/default_schema. The com.system76.CosmicPanel directory contains a key called entries, which is a list of profiles to be loaded. Each profile then has its own directory, for example, com.system76.CosmicPanel.Panel. You can make changes to the keys in this directory to alter the config. After making changes to copies of the provided config in data data/, you may install each to $HOME/.config/cosmic/ find data/default_schema_copy -type f -exec install -Dm0644 {} {{$HOME/.config/cosmic}}/{} \;

cosmic-panel

See the following for examples of applets and plugins which can be installed and used:
https://github.com/pop-os/cosmic-applets