4-inch to 3-inch Adapter for Spray Booth vent outlet

I've been wanting to learn airbrushing on small models for a while now, so I bought a spray booth that has a 4" vent hose to vent paint fumes outside. (Search on Master Airbrush Spray Booth)

We have a bathroom in our basement that does not get much use, so I set up the spray booth there, and designed an adapter to connect the 4" hose to the 3" ceiling exhaust fan outlet which vents to the outside of the house.

The adapter is in two pieces for easy printing without support. I sanded the mating surfaces and glued the two halves together with epoxy, letting it cure overnight.

I removed the bathroom exhaust fan grill and then removed the motor/impeller unit, so the adapter could be installed.

Since the exhaust fan housing is made of sheet steel, I added two 1" recessed areas to the adapter for gluing in some round magnets to support the weight of the adapter and hose. The 3" outlet on the housing was about 2mm below the top of the housing so I made the areas for the magnets 2mm deep and used 4mm thick by 1" diameter ceramic magnets. The ceramic magnets turned out to be strong enough to support the adapter - you could use neodymium magnets and get even better support.

A thrust plate on the 4" section of the adapter allows for a piece of hard foam to be wedged in place to keep the 3" end of the adapter firmly against the outlet.

The 4" x 6' hose that comes with the spray booth was not long enough for my needs, so I ordered a 10' dust collection hose from Harbor Freight. I used the rubber collar that came with the booth to attach the hose to the adapter, then used a 4" hose clamp to attach the hose to the flange that attaches to the spray booth. It leaked some air around the hose clamp, so I also put some duct tape there to seal it well.

The 3" part of the adapter has a slot in it to accommodate a long screw that was used to attach the bathroom fan housing to the hose that goes to the outside. Your fan unit might have a similar screw(s) but likely in a different place. You could probably just cut off the slotted portion of the adapter in your slicer before printing it, and it would work fine as long as your fan housing is 3" diameter. Some tape or silicone can be used to make a good seal where the adapter fits into the fan housing.