The aux battery in the F15 is a smaller H4 battery class size and there’s the PCU (power control unit) that sits between the main battery and aux battery.
Installing a larger H8 or H9 battery next to the main battery would present a challenge you seemed to have conquered…you had to be creative to make it fit.
Hopefully you added the additional VENT TUBE (and the little black elbow that connects the vent tube to the battery) so that you can vent any off gasses from the additional battery to the exterior of the car (#9 & #10 in illustration below). . And if you haven’t thrown the cover from the battery posts on the new battery…make sure you pop off the additional vent plug so that you can insert it into the other vent hole on the new battery to prevent off gasses from venting from the other side of the battery that is NOT connected to the vent tube.
You can see the holder (#5) and the clamp (#3) in the illustration below that holds the aux battery and PCU.
This is the PCU & additional holder that sits between the main and aux battery:
