I can't believe in the 5.5 Volts as the lower bound. When you are using the d-Li90 they camera says low at 7.1V with the K3 and 6.8V with the K1. I consider Voltages below 6.8 as dangerous, there may be accidental picture / function loss, switching of of the camera. Therefore I would set the lower bound at least to 7.2 Volt.
The external power supply which can be connected ist 8.4 Volts roundabout 2 Amps (
Dot.Foto-Netzadapter ersatz für Pentax K-AC50: Amazon.de: Elektronik ). The Pentax Input specification is 8.4 Volts
There are smaller and cheaper Voltage regulators available at ebay, there ist one with adjustable outputvoltage ( Input 3,3-40 V Output 1,25-35 V DC/DC with an LM2596S) which should fit
LM2596S Einstellbarer DC/DC Step Down Spannungsregler Modul für Arduino ESP8266 | eBay Your Input should provide more than 2.4 Amps continuously to the regulator - therefore some USB Powerpacks won't fit, because the clamp near 2 Amps. I would set the voltage to 8.4 Volts. You could easily modify a batterie grip, especially the Battery-Pack, I would feed it with 6 external Batteries D-Type (roundabout 5 Ah 3 times the D-Li90 capacity) or a bike Batterie (12 Volt 18 Ah) with an voltage regulator.
There is a thread at a german photoforum
Netzadapter mit 8,4V bauen - DSLR-Forum