Adapter for using 3M 6000 series filters on 40mm NATO masks, fits most of the old soviet ones.
Can pick the filters up for nothing on eBay or from local hardware shops, B&Q has plenty.
You'll want a 40mm o-ring to make a proper seal.
Thanks to 3DModularSystems for the 3M fitting on this model: https://www.thingiverse.com/thing:1487286
And to uumeran for the 40mm NATO thread: