The stock 2002 Honda airbox is perhaps the weakest link in the production of the CR125, CR250 and the CRF450. A good idea when prepping any new bike, is to re-seal the airbox to airboot junction, by using a good silicone sealant. But in this case, the Hondas have an especially weak airbox seal; nearly to the point where it can't really be fixed properly. PC Racing developed a kit which provides a gasket to install between the airbox and the airboot, along with a thick, foam-like seal to be installed where the air filter seats to the airbox. The foam seal also alleviates another problem regarding the Honda airbox, which will be explained below.

Remove the subframe assembly, separating the airbox from the subframe.
Now disassemble the airbox assembly, which consists of the airbox, backing plate and the airboot.
It is very important to completely remove the factory glue from the sealing surface between the airbox and the airboot. Do this by using an adhesive remover such as a 3M product. Contact cleaner will work as well.
Now that the surface is clean, its time to install the gasket which goes between the airbox and the airboot (the thinner gasket of the two). First be sure that the mounting inserts are no higher than 3mm (or 2 dimes.) If they are higher, trim them down with a razor blade.
Place the gasket on the airbox, stretching the holes over the inserts as shown.
Next place the new machined aluminum backing plate over the airboot. Then attach the airboot to the airbox via the stock screws. Take extra care to not strip the screws; remember that you are using a metal screw in plastic.Tighten each screw down in a cross pattern, until each is snug.
The other thicker gasket serves two purposes, in the 02 Honda's case. 1) It allows for a good airfilter to airbox seal without the use of grease. 2) Solves the sealing problem as a result of the protruding airboot mounts as shown in the photo.