Energizer Ultimate Lithiums or Eneloops in a battery grip. You can save money on a replica battery grip here:
Battery Grip For Pentax K7 K-7 DSRL D-BG4 - Newegg.com
Not as good as the original, but close enough (I have one of each). The replica is close enough that I can swap battery trays between them. Take the money you save on the BG to get 2
additional sets of batteries and/or D-LI90s, just to be sure.
In any case, you should always carry around 2 new sets of Ultimate Lithiums for emergency backup.