Adhesive-backed 1/4" or 5/16" weather stripping from a hardware store
There has been much debate as to the value of this mod, but I see no way that it can do harm. It should, at the very least, be worth a couple of horsepower at very high speed.
On stock 99 and newer Milles, the ram air ducts are not sealed to the outside of the frame, but rather have a small gap at the point where they join to the frame. For reference, look down the front of the air ducts, while shining a flashlight at the air ducts. These gaps allow pressurized air to escape instead of being forced through the air filter and into the airbox. Quite simply, what you are sealing is the ram air ducts to the frame. There is about a 3mm-8mm gap there right now. The airbox is already sealed to the inside of the frame quite well.
All you have to do is remove the mid fairings, and then the inner and outer ram air ducts. You have to use a small Phillips to separate the two halves of each tube prior to removal, and remove three 8mm hex head bolts for the inner air duct half (remember all the bolts). Then, you cut to length a strip of adhesive weather stripping (Lowe’s or Home Depot) about 1/4" or 5mm thick and wrap it around the intake air opening on the frame. You should not see the weatherstripping from the outside when you are done. Assembly is the reverse of removal, but may be more difficult due to the added thickness of the weather stripping. For the really bored, you can actually seal the two halves of each air duct with non- drying silicone for the maximum benefit.
What you are trying to do is expand the effective total volume of the airbox, and provide the potential for an infinitesimal amount of boost at very elevated speeds. This will definitely not cause any problems in 99-02 bikes as my 1999 Mille is now at nearly 25K miles with this mod in place.