you should've gotten the 9.6v L battery specifically for p90s
What you will need is the extended rubber buttplate that replaces the slide hatch plate. Depending on how much battery there is vs how much slide plate, you could dremmel a little out, but it's not recommended.
One thing to check is at the bottom of the battery compartment (with the business end pointing at the floor), there is an extra lip you can break off that won't effect the strength of the body, this will get you a few more mm to work with. There's a thread I believe somewhere on asc that shows how to get rid of it.. you need to modify a screw driver or find a slotted one to get in there and straddle it to break it off.