I wanted a good way to not include the folder name in the database entry for Nova images, and also be able to track a file on any filesystem. Image name is based on the actual filename. Use at own risk, I made this for a personal project.

NovaTrackedExternalImage.php

<?php

namespace Dongo\NovaTrackedExternalImage;

use Illuminate\Http\Request;
use Laravel\Nova\Fields\Image;
use Storage;

class NovaTrackedExternalImage extends Image
{
    private $prefix = false;
    private $download_disk;

    public function __construct($name, $attribute = null, $disk = 'public', $storageCallback = null)
    {
        parent::__construct($name, $attribute, $disk, $storageCallback);

        $this->download_disk;

        $this->preview(function ($file) {
            return $this->prefix . $file;
        })
            ->thumbnail(function ($file) {
                return $this->prefix . $file;
            })
            ->download(function ($request, $model) {
                $name = $this->originalNameColumn ? $model->{$this->originalNameColumn} : null;

                return Storage::disk($this->download_disk ?? $this->disk)
                    ->download($this->storagePath . '/' .  $model->{$this->attribute}, $name);
            })
            ->storeAs(function ($request) {
                /**
                 * @var Request $request
                 */
                return optional($request->file($this->attribute))->getClientOriginalName();
            })
            ->store(function ($request, $model, $attribute, $requestAttribute, $disk, $storagePath) {
                /**
                 * @var Request $request
                 */

                $originalName = optional($request->file($this->attribute))->getClientOriginalName();

                optional($request->file($requestAttribute))->storeAs($storagePath, $originalName, $disk);

                return [
                    $this->attribute => $originalName
                ];
            })
            ->delete(function ($request, $model, $disk, $path) {
                if (!$path) {
                    return;
                }

                Storage::disk($disk)->delete($this->storagePath . '/' . $path);
            });
    }

    public function prefix(string $prefix)
    {
        $this->prefix = $prefix;
        return $this;
    }

    public function downloadDisk(string $disk)
    {
        $this->download_disk = $disk;
        return $this;
    }
}

useage.php

<?php

namespace App\Nova\Resources\Dongos;

use Dongo\NovaTrackedExternalImage\NovaTrackedExternalImage;

class Dongos extends Resource
{
  public function fields(Request $request)
  {
    NovaTrackedExternalImage::make('Image (filename is the image name)', 'Image')
                    ->disk('dongo_local_public')
                    ->downloadDisk('dongo_public')
                    ->prunable()
                    ->path($this->storageLocation)
                    ->prefix('https://www.dongo.com/images/subdirectory/'),
  }