Build NuGet package from external binaries

I have to use external binary files in my .NET Core 3.1 project with a file structure like this

external.dll
external-dependency.dll
some-useful.exe
resources/
|-- some-resource.ilex
|-- some-resource.bin

To avoid inserting those files into my git repository, I want to package them into nuget. At the moment my nuspec file is looking like this:

<?xml version="1.0" encoding="utf-8"?>
<package>
  <metadata>
    <!-- Some package infos here -->
    <references>
      <group targetFramework="netcoreapp3.1">
          <reference file="external.dll" />
      </group>
    </references>
    <dependencies>
        <group targetFramework="netcoreapp3.1"></group>
    </dependencies>
  </metadata>
</package>

I'm not sure how to embed the resource files into my project, I have to specify the resource path when I'm using some classes inside the external.dll. If I am using <contentFiles /> inside the metadata, the assembly reference is missing:

Without the contentFile structure, I'm getting this result after the nuget restore:

I hope that sombody understands my issues. The external company does not have published any NuGet-Packages yet and they wont do it in the near future.

Greetings