First, I started with a DRZ400 battery. I guess a CRF450X battery would be about the same size, but my son has a DRZ so I had the battery available.
I placed the battery on top of the airbox and traced where I was going to cut out.Next, I slid the battery into the new cutout. The battery needed to go down another inch, so I cut a chunk out of the back of the airbox and the battery slid right down to where it needed to be.
Then I used some 1"x1/8" aluminum flat stock to create a cradle for the battery to sit in. I rivited it to the front and back of the airbox.
I still had the problem with the back of the battery hanging out the hole I created in the rear of the airbox. I used a piece of black plastic from the corner of a paint rolling pan to cover the hole.
I used copious quantities of silicone and some rivets to hold it on. It doesn't leak,it looks good, and you can hardly tell that it's there.
Next I drilled lots of holes in the top of the airbox. I figured that it would be better to have the holes on top rather then in the side of the cover in case I went through deep water.
Next, I rerouted the wiring and electronics into the airbox. You can see the cradle I fabricated out of the aluminum stock.
I just slid the battery in and hooked it up.
Side view with battery installed.
Finished, with XR600R side cover on.
The seat did not have to be cut or compromised in any way. The air filter is easy to get in and out. The holes I drilled in the top of the airbox allow plenty of airflow and I haven't had any trouble in water so far. The weight is off of the rear and more centered. All in all...a very positive experience