Feasible to build? I smoked rollies with cardboard filters for a decade; each "built" myself.

As for the toxic chemicals, I am also strongly in favor of mandating tobacco-only cigarettes; ones without the cancerous cocktail of additives designed to maximize addiction. That would not only take care of a massive chunk of toxic chemicals but have the very likely effect of making quitting easier for addicts.

As for such a mandate "encouraging littering" - mitigating harm from littering doesn't encourage litter, that's like saying getting rid of plastic bags encourages littering of paper bags. It's such a weird point to even try and make.