Trying to base a mod off of portal2mod.esp, seems to work all fine and stuff in the CK, but it doesn't save resources used from it (In this case Space Core)
I've tried using TESsnip to set it as a master, and the CK just removes that when I open it up to re-add the Space Core.

The CK strips all ESP masters while saving a plugin. To workaround this, you'll have to use Wrye Smash to 'ESMify' them before loading your plugin.