I have my printer enclosed so I cannot easily see the spool to check how much filament is remaining - also lazy. After pondering a little bit I realized that it would be pretty simple to add a cheap load cell to the printer, put the spool on it when new then count backwards from 1000 grams to tell how much filament is left on the spool. A 5kg load cell with a HX711 amplifier board is under $10 on amazon. I looked around and didn't find anything already made. All the necessary libraries for python to utilize a HX711 are extant.
It should be pretty easy to write an Octoprint plugin to add spool weight monitoring. I would do it except for two things - I don't know python, and I don't want to learn python.
Here is where the project is at: the spool holder is designed and works fine to hold a spool of filament. I have been printing off of it for a few weeks. All that is needed is software to interface the load cell to octoprint. I am tossing this up here hopes that someone with the necessary python skills will be interested enough in this existing to write the software then we can put it together into a plug in.
The stl files and pictures of the first design are up on thingiverse: