The cheapest technical solution might be to offer them a [vaporizer][1]. There are low-tech, efficient ones called "vaporization pipes":

[![A passive-convection "vaporization pipe" with flame filter ][2]][2]

I've seen some at parties a long time ago. People were smoking right next to me and it smelled as if they were drinking tea, nothing more.

You'd have to ask your neighbors first but those pipes have objective advantages for the users too:

* No need for tobacco
* Much easier on the lungs than other inhalation methods
* More efficient because THC isn't destroyed by pyrolysis during smoking.

As far as I can tell, it is legal to buy and own such a pipe. What your neighbors put inside is not your problem.

It might not be the solution you had in mind but it might be worth it to try it for $50 before investing $XXXX in other technical solutions.






  [1]: https://en.wikipedia.org/wiki/Vaporizer_(inhalation_device)#Marijuana_vaporizers
  [2]: https://i.sstatic.net/c4M7L.png