How to make custom View-Master 3D reels

The View-Master company has gone through some changes over the years, now acting as a pseudo virtual reality company. But those of us who grew up with the iconic View-Master toys knows it better as a 3D photo company. I was thinking about how View-Master viewers have retained their value over the years -- not surprisingly -- because the basic elements needed to enjoy a photo reel/disc has been relatively unchanged. While the shape of the viewer has gone through some iterations of its own, the concept of slipping a circular disc into the viewer to be transported to any given subject has remained quite fun.

So I was wondering if it was possible to make a custom reel. It must be possible right? It is!

One company does it but with a crazy expensive set-up fee. So that's no good. The other one I found seems to be much more affordable. Mind you, I haven't tried either of these so if you are going to jump right in, do so cautiously. But it does seem like both of them are able to make any 7 images you choose 3D. I might have to give this a try come Christmas. :)