I'd suggest a replacement filter box with good clips then to coer all eventualities. There's a couple of late 940s being broken at present :
https://www.volvoforums.org.uk/showthread.php?t=287440
https://www.volvoforums.org.uk/showthread.php?t=299462
First one is 360beast (Luke) and second is Griston64 (Mark), a lot of the stuff has now gone from both cars but i think the air filter boxes are still there.
Meanwhile until you get a replacement, something like this would do to hold it closed :
https://www.ebay.co.uk/itm/Luggage-s...r/274095159153
Not sure on the size, definitely worth measuring instead of guessing as it's surprising how all the sides add up to the toal length!