you can build a cat box enclosure and have powered ventilation to the outside.
The components for the exhaust are readily available from marine supply sources. Photos of the components following the text.
You need an ABS plastic 3" louvered vent. They come in white or black. The louvers keep the rain out, for a van I strongly suggest cutting a piece of bug screen to glue just behind the louvers. You would need to put a hole in your van for the vent to go through. Even if you later did not have a cat you can use it for an exhaust fan or for a small fresh air source.
For the power you buy a 3" bilge bilge blower, they cost as little as $20.00 from various online sources. They run on 12v and don't use a lot of power. A section of tube connects the blower to the inside edge of the louvered vent. The size of tube you need is also available from the marine supply sources. All these items are designed to connect together for exhausting fumes from the bilge compartment on boats.
Remember that in order for air to exit you must have some type of opening in the compartment for air to enter.
Turning it off and on. Connect the power to a motion sensor with a time delayed shut off. It goes on when the cat enters or you can have a delayed start, then it shuts off at whatever time period the unit is built with or if customizable time they can set it for what you want. Or of course you can use a manual switch to turn it off and on. But in the middle of the night a sensor means you don't have to get out of bed.
For size reference on the photos remember that the tube coming out of the blower is 3" as is the tube that is on the back side of the louvered vent cover. These units are sturdy and made to be in salt water conditions, they will last a long time. On boats they are also used to vent the galley area to remove cooking odors.