You could probably increase the 82K and 10K resistors to be much bigger (by a factor of 10x or maybe even 100x). Lookup the input impedance for the ADC of your model of ATmega, as long as it’s >10x the size of your resistors then your circuit will probably be accurate enough.
A couple more things to keep in mind:
a fresh alkaline 9V battery is actually 9.6V or more, not 9V.
9V battery voltages droop noticeably when under load because of their high internal resistance. Make sure to measure under the same conditions.