How low are you able to get the temp on your kegerator?
If you are doing a 2 liter bottle, why not stick it in the freezer. At 32 F, 40 PSI will get you 6 volumes. Of course I don't know what happens to a carbonator cap with that much pressure when it warms up - could be messy.
"Our results are merely the result of carefully managing the transformation of bee spit into yeast excrement."