Red light filter with a 3D printed, friction fit cap.  A hand cut lens sourced from Amazon.  Badabing!  Now you have visible red light on your white light filter for cheap. 

We sell it, but you could easily make it (no lie).  In fact, here's a download link for the file (for free!):  

Here's an affiliate link to the lens we used:  https://amzn.to/4eIG0gw

So make them, or just be lazy and buy them from us and be done with it.