Hi, I'm the author of the post. We use this hack on the bridge of Navy ships. The red density required is pretty significant (the plastic we used was a little thinner than a #2 pencil). We would velcro it over the screen. The velcro leaves a gap which creates a fairly significant light leak around the edges. First iteration, so better, but a long ways to go. Comparing that to Blacktree's nocturne, software is at least a significant part of the solution.

I've personally seen astronomers use red perspex covers for their laptops/phones as a way of preserving their night vision in the field.

I imagine that there could be some edge cases where contrast could be low etc. But it's a cheap solution.

Some amateur astronomers have red-lens glasses for this purpose. Also allows them to run inside a lit house without spoiling their nightvision.