Revisited this idea with some new parts. I bought a Buck converter. Sure, you can make one: There are all-in-one IC's available that only require a few passive components to make the circuit. But, I like the quick solution (and cheaper one too).
I found an extra Ryobi charger in a junk pile. I gutted it. Soldered new copper wires to the battery terminals, added a double-pole-single throw switch at the battery end. Then, added this nice DC-DC step-down converter (buck) with output current and voltage displays. The converter has an adjustable output 5-30V with an input range of 0.8-32V. Load regulation is speced at 0.5% which is close to the actual performance.
Why a buck vs. a linear regulator? For efficiency. I have a couple Lithium-ion drill batteries which are 18V. My Porta Pi Arcade needs 5V. Linear regulars dissipate the excess voltage through heat. Buck converters (step-down converters) switch the power on and off to provide power to the load. Buck converters are much more efficient and converting power. Making the most out of your battery source.
I really like the output display. I milled out a view window so you can see the output. The rest I did with a dremmel (which got away from me a few times).
I bought the step-down converter on ebay. It's one of those Chinese "gadgets". It is spec'ed to output up to 5A. I cant see the marking on the IC's and just to be safe I dont expect to sink more that 3A from this. Just to be safe. I've tested my luck running close to the written specs on many of these DIY circuit parts and I've always lost that bet around 90% of the listed limits.
It works rather well. A great mod you can do for anything application. The converter was $11.xx shipped. I also bought this beefy MeanWell buck converter which is also listed for max output of 5A. Now you tell me which one of these converters is lying about their max ratings?
[update] I wired up the MeanWell converter and ran the Porta Pi with a fully charged battery. Total run time was 2hr 45min with the volume set to a whisper.