Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:4444335.
Note: This design requires the use of flexible material for the 'insert' and 'gasket'. Optionally use a rubber gasket (garden hose gasket works well) or actual 3M mask washer for the gasket. A user has reported than printing the 'insert' at 97% with rigid material also works with minor fitting on the adapter part.
This adapter provides an attachment point to add a filter to the exhaust valve of a 3M 7500 series half mask. Normally, exhaled air is simply exhausted directly to the surroundings via a one way valve. This is perfectly acceptable when using the mask to protect oneself from the surroundings, however it does not necessarily protect the surroundings from the user.
This adapter fits into the exhaust slot in the mask and provides a 3M bayonet attachment point for a filter. You can use any of the designs that fit to the standard 3M port, but I recommend something on the smaller side. I use my mini filter, but there are plenty out there: https://www.thingiverse.com/thing:4219678 If your filter material has a preferred flow direction, keep than in mind when constructing your filter, as the airflow through this is the opposite direction of a normal filter use scenario.
Updates: 2020-06-30 - new versions of insert and adapter uploaded that are slightly larger and more draft angle to provide a more snug fit into the mask