Test complete…

One old APC UPS, wire, 20A automotive fuse, pliers and some electrical tape. To acquire later: 2 used SLA automobile batteries, preferably 60A/hr or higher rating∗.

Prelimary test, 12V 6A/hr SLA battery.
Live test on a 12V 80A/hr SLA Battery!

∗ To calculate runtime, use A/hr rating x 12V = total wattage, divide by power consumption, allow 33% margin for inefficiencies plus you don’t want to run down a SLA (Sealed Lead Acid) battery which is a big no-no! For 80A/hr battery, you get about 960W which if you’re running about 100W of stuff, you should have at about 6hrs runtime.