I never use those boxes.
A laundry sack is ideal, as you can tie them closed but the beast can still breathe through them. The cat also is generally happier if it can feel your lap under it as you drive- and if it's being a hissy thing with sharp bits, you can simply carry it at arm's length and there's nothing the cat can do about it.

It will definitely get you some odd looks, though...
( The Resident Loon Not a demographic. Do not measure., Mon 5 May 2008, 12:57, Reply)