WordPress / Media Grid 5.1+: Route 404 Featured-Image URLs to a Default Placeholder

Four-year-old uploads that my Media Grid (version 5.1+) relied on were wiped during a recent storage cleanup. Each affected post now breaks the grid because its featured image URL returns 404.

I need a clean, future-proof fallback so the existing items stay intact without manual re-creation:

• All missing featured-image requests should seamlessly display one default placeholder image I’ve already prepared.
• Live posts, categories, and the grid layout itself must remain untouched; only the thumbnail output or URL handling changes.
• The solution can be a smart .htaccess rewrite, a PHP filter inside a lightweight plugin/snippet, or any other approach you know works best with Media Grid’s internal calls—whatever achieves the fastest loading and least maintenance.

Please bring hands-on experience with the Media Grid plugin (especially post-v5.1 changes) and WordPress hooks so we avoid plugin conflicts and keep the fix upgrade-safe. Let me know how you plan to implement it and how I can test that every missing image now shows the placeholder while existing images render normally.

GOAL: Goal: Keep posts/grids intact; only swap missing featured-image output via .htaccess or lightweight PHP filter; upgrade-safe and fast.