I made my own 12V UPS a few years ago after having little luck finding a reasonably priced alarm/CCTV battery backed panel at the time. I bought a small 12v 6Ah LiFePO4 battery (built-in BMS), a 15V power supply and a buck converter with CC and CV controls (adjusted to 2A and 13.5V). I made up a small enclosure a fuse for the battery, DC input and two leads with DC plugs that have kept both our 4G router and VoIP based FritzBox running for a few years now, including a few multi-hour power cuts. The buck converter's LEDs which glow through the enclosure effectively provide a status, i.e. Blue = On mains, Green = On Battery and Purple = CC, i.e. Mains + Recharging.
I see these PSUs all the time, definitely an old CTEC and were great back in the day. The 317 is almost always the first point of failure and is noticeable if your output voltage drops to around 9v. Keep an eye out for Elmdene 1381 or 1382 (rated 1a and 2a output load respectively), they’re great little boxed PSUs and the newer versions are switch modes, not majorly expensive new either. I’d probably avoid the CCTV power supplies as under larger loads they are prone to be very electrically noisy and don’t have battery backup.
You could have dropped the voltage with a coupe of diodes in series i've just invested in a couple of 'talentcell' battery banks, one has usb and 12v, one has usb, 9v and 12v, one of them is called a 'ups' which has 2x usb, 9v, and 4x 12v, i've made vids on them ;)
