Evening,
I had a look around to see if there are reducers/adapters to accommodate a smaller vent or accessory (to decrease the size of the opening) however failed to find anything useful. It can definitely machined from PVC or aluminium so worth contacting some machinists or people offering their
CNC services on eBay.
Alternatively, you could DIY one. By using little filler like the one linked below, you could stick some quality sheet metal over the hole left by the skylight and have the correct sized hole cut in said sheet metal. All that’d be left is riveting it on and sealing over the whole thing.
Note, this may or may not work with your van. may want to check the size of the ridges on your roof and find a correct one.
For the Fiat Ducato Peugeot Boxer & Citroen Relay only. SKYLIGHT Roof Vent INFILL PROFILE BLOCKS. Always always use a quality sealant in all areas of roof vent installation. Your roof profile is approx 8mm deep.
www.ebay.co.uk
Lastly, take care whilst working on the roof.
All the best!