Unity provides the SettingsProvider API to create custom project settings. Additionally, there exists the ScriptableSingleton class to create a scriptable objects that can be statically referenced and kept outside the assets folder. This combines both APIs, to create custom project settings in the editor without the hassle.

ExampleProjectSettings.cs

[FilePath("ProjectSettings/" + nameof(ExampleProjectSettings) + ".asset", FilePathAttribute.Location.ProjectFolder)]
public class ExampleProjectSettings : SimpleSettings<ExampleProjectSettings> {
  /// <summary>
  /// the display path in the project settings window
  /// </summary>
  private const string WINDOW_PATH = "Tools/" + nameof(ExampleProjectSettings);

  public bool exampleSettingBool;

  /// <summary>
  /// Register the settings provider for the project settings
  /// </summary>
  /// <returns></returns>
  [SettingsProvider]
  public static SettingsProvider RegisterInProjectSettings() {
    return new SimpleSettingsProvider<ExampleProjectSettings>(WINDOW_PATH);
  }
}

SimpleProjectSettings.cs

using UnityEditor;
using UnityEngine;

public class SimpleProjectSettings<T> : ScriptableSingleton<T> where T : SimpleSettings<T> {
  public void OnEnable() {
    hideFlags &= ~HideFlags.NotEditable;
  }

  public void Save() {
    base.Save(true);
  }
}

SimpleProjectSettingsProvider.cs

using UnityEditor;
using UnityEditor.UIElements;
using UnityEngine;
using UnityEngine.UIElements;

public class SimpleSettingsProvider<T> : SettingsProvider where T : SimpleSettings<T> {

  private SerializedObject serializedData;
  private string headline;

  public SimpleSettingsProvider(string path, string headline=null, string[] keywords = null) : base(path, SettingsScope.Project) {
    this.headline = headline ?? typeof(T).Name;
    this.keywords = keywords ?? (new string[] { typeof(T).Name });
  }

  public override void OnActivate(string searchContext, VisualElement rootElement) {
    serializedData = new SerializedObject(SimpleSettings<T>.instance);
    CreateHeaderAndAddStyles(rootElement);

    // Create a scroll view with a inspector inside
    ScrollView scrollView = new ScrollView(ScrollViewMode.Vertical);
    InspectorElement inspectorRoot = new InspectorElement();

    scrollView.Add(inspectorRoot);
    rootElement.Add(scrollView);

    // Bind a callback to save changes
    // Binding it to the root is enough
    rootElement.RegisterCallback<SerializedPropertyChangeEvent>(ValueChanged);

    // Populate the properties
    inspectorRoot.Bind(serializedData);
  }

  protected virtual void CreateHeaderAndAddStyles(VisualElement root) {
    // create and add label
    Label title = new Label(headline);
    title.style.fontSize = 20;
    title.style.unityFontStyleAndWeight = FontStyle.Bold;
    title.style.marginBottom = 10;
    title.AddToClassList(nameof(title));
    title.style.paddingTop = title.style.paddingRight = 2;
    title.style.paddingBottom = 0;
    title.style.paddingLeft = 5;

    root.Add(title);
  }

  protected virtual void ValueChanged(SerializedPropertyChangeEvent evt) {
    (serializedData.targetObject as T).Save();
  }
}

Thanks to the Unity Essential Success team for cooking up the classes this solution extends upon.
Nonetheless, please be aware that this code has not yet been battletested.