There is an article on ADBMAG.COM
that has some info on the 07 WR airbox & issues. Extract just on the airbox as follows:
PROBLEM: To accommodate the new alloy frame and subframe on the '07 bike, the WR450F's airbox was also modified. But unlike the brilliant '06-model airbox design, the '07 WR450F's airbox is average and the bike's filter clogs up more quickly.
It's not so much an issue when riding behind other bikes in dust; it's more an issue of sucking its own dust. Literally, after a few laps of slamming dusty berms, the filter is filled with enough dirt to noticeably affect performance. Sure, the WR-F's airbox now comes with a handy
hinged door and easy-access clip-in filter cage, but an easily fouled filter is not ideal in any conditions, let alone while Australia is in the midst of a 100-year drought! Our diagnosis: too much dusty air was being drawn in from under the cavernous gap at the rear of the seat.
SOLUTION: Initially, we simply fitted a layer of coarse foam under the rear of the seat, to act
as a kind of pre-filter. But this only partially addressed the problem. As Geoff Ballard explains, "It's not so much that the new set-up is bad; it's just that the '06 model was so good. In any case, we found ourselves having to clean our air filters more on this year's bike, so we started to look at how we could neutralise the air pressure in the airbox. When you look at the airbox on some Suzuki
models, you'll see they have a huge opening at the bottom of the airbox, but their filters still don't get dirty as quickly. So we started experimenting with a similar design on the new WR-Fs; the idea being that letting more air in the bottom would reduce the vortex
or tunnel-effect through the top of the airbox." Ballard experimented with it on his race bike, with good results.
All you need to do is remove the air filter and cage, then drill four or five 20 to 30mm-diameter holes in the base of the airbox with a stepped cone-drill (these drills are around $100 a pop, so you could improvise with another drill bit or a Stanley knife). You then cut a small rectangle of coarse GYTR
foam and lay it in the base of the airbox. The whole mod takes about 10 minutes. It doesn't make it as good as the '06 bike but it does significantly reduce the frequency you'll need to clean your air filters. Also, you'll notice that the dirt is more evenly spread across the entire surface of the filter, rather than being concentrated in a few areas – in which case, you run a greater risk of creating a dry-spot in the filter foam and possibly sucking dust."
..for reference, Geoff Ballard is owner of Ballard's Offroad in Australia and runs a race team of 7 WR's from memory