From the onset, I focused on three things. Battery life, portability/weight and sound quality.
I researched more than a few existing products on the market and while some of them looked promising, they missed the mark with one of two things. Either the battery life was short or the sound quality was sub-par.
Step 1: Battery life
I decided on a sealed lead acid battery (SLA). SLA batteries, when used properly, can provide years of service. They are sealed and maintenance free. They are very popular in many low tech applications such as motorized scooters, wheelchairs, uninterruptable power supplies, etc .
Choosing an SLA battery is as simple as picking one based on the amp hour rating. The amp hour rating is generally expressed as nnAH where nn is a number designating hours. In my case I used a 20AH battery. What this means is that the battery, under ideal conditions, would be able to provide 12 volts for 20 hours, to a device drawing one amp of current. I'm not an electrician, so I may not be wording this technically correct.
Choosing a lower amp hour battery would reduce battery size and weight, at the expense of run time.
I chose a Battery Tender Junior battery charger which both charges and maintains the battery when not in use. SLA batteries should be stored fully charged and if possible attached to a charger that can maintain the charge. Another option is to use an onboard charger which is meant to be installed into whatever the application may be. The Schumacher 1.5A Onboard Charger gets great reviews.