I've had to venture below the airbox a few times now in my S40 2.4i. Now I may be clumsy, but every single time this blasted thing has been the hardest part of the repair by miles.
Most recently I was heading under there to take a look at a sticky starter (which seems to have solved itself for now). I did what avenger09123 suggested - cut off the leftmost "arm" holding the box, which was a hugely helpful maneuver, I was able (just) to remove the box without shifting the engine.
Though I got the box out, I severed a coolant hose doing so (my fault), and was generally fed up with its bulk and how tricky you had to be with it and everything around it to coax it out.
Enough was enough for me, so in the interests of serviceability and not noise, MPG, power, or any other dubious claims that come with an intake:
This is a Spectre 9833 inline air box with two silicone elbows (trimmed a little), a Spectre 87051 MAF mount which seems to have our stock MAF tube's inner diameter (simpler than the similar Spectre 9405 kit), and a bookshelf bracket holding the ECU (subject to improvement, hopefully). Other things include a 3" coupler and a 10mm breather filter, which clamps around the breather line with the OEM 90 degree elbow removed from it. Also: sink strainer mesh before the MAF tube to emulate the pre-MAF mesh in the OEM box. It's a good hint from a member on here, from the Cheap Mods thread I believe. I had a lumpy idle and hesitant acceleration before doing that, silky smooth now.
Oiled gauze air filters are notoriously less than ideal. I'm using foam as a pre-filter (oiled lightly with foam-specific stuff) to hopefully shore up the filtration a bit on both the airbox filter and the breather filter. You can see it on the breather filter, I did something very similar with the one inside the airbox.
It's quiet until 4k RPM! This is why I chose the relatively small diameter airbox - didn't want any din from an oversized one. Ironically in my case it's quieter than the stock setup at low RPM, as the "elbow" going to the snorkel was rattly in my S40. Per Spectre, the CFM rating (of the box, not just the filter itself) should be well over what's needed for the 2.4i, and I'm certain I haven't lost any appreciable amount of power.
Figured I'd share an inexpensive way to throw together a very simple intake for the 2.4i. No codes so far, but I'll update with developments if there are any.