After looking at Century options and a few others, have decided on another Australian company, SSB (Super Start Batteries) SS55TI, this should be similar to what some do in the US with the X Power D4700.
Direct replacement would be the SS55 600CCA, but the SS55TI is slightly taller (but as...
Same has happened to me, if you leave the car in Full Guard mode for 2-3 weeks, the battery will go flat, if parked in your garage, it is best not to use Full Guard
I ordered mine last week for A$11.21, now A$11.91 today
https://www.aliexpress.com/item/TRUNK-ENVELOPE-STYLE-CARGO-NET-FOR-FORD-MUSTANG-2015-2016-BRAND-NEW/32718629852.html?spm=2114.01010208.8.22.rnxhfW