As an aggregator you could use Wago (left on picture above) connectors (they should withstand the current, despite they are not rated for it) – but I’ve used connector (two on the right side of the picture above) design to be used in mains line boxes rated for 80A. Printable version of battery holder and the box itself I’ve put on Thigiverse. This is assembled version printed in PLA with “old” type switches

The charger also has a belt clip that feels strong but I didn't use this. Honestly I think it would hold up to 5ah batteries, it's just too bulky to have in your belt and walk around. The figment was very tight to the Makita 5ah battery I tested, actually nice to see this since some brands have very loose connections. No issues removing or connecting the battery.It’s part of Makita’s expanding 18V Lithium-Ion system, the world’s largest cordless tool system powered by 18V Lithium-Ion slide-style batteries. Makita 18V Lithium-Ion batteries have the fastest charge times in their categories, so they spend more time working and less time sitting on the charger. Schottky diode – 25V+ 20A+ (I’ve used 20SQ045 – 45V, 20A – bigger would be better, this is what I had at hand) You should also try to, as precise as you can, set up every step down converter to the same voltage levels. Recommended load is 15A (small print), so around 180W. I’ve prepared additional cooling, so I guess it should manage to go to 20A peak at 13V (240W-250W). Makita battery connectors – you could also print it. I’ve manage to find genuine parts in Poland for 2EUR

You can help yourself with the picture on the top, where there is everything lying on the floor and described components. Just add switch and breaker box. Warm Reminded: The max power of this power supply is 200W, so please make sure that the power of the electrical appliances should be less than 200W of the rated power of the inverter

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Compact LED Nightlight: Featuring a 450LM LED work light, with its compact size (4 x 3 x 4in) and lightweight 0.7LB, this portable battery inverter is a great gadget for outdoor power solution and outdoor camping light, and you can store it anywhere To connect battery holders with the box itself, you should have some M3, 10mm (perfect length – can be longer) long bolts and nuts inverter still requires at least 67Amps (more like 80A) for 12V and this is already a lot of current (twice at peak). For large car batteries it’s not something terrible (my 100Ah battery has peak current over 800A) – but it could drain it real fast, and four Makita batteries in a blink of an eye. Powered by (1) 18V LXT® battery with the option to insert (1) additional battery for extended run time (batteries sold separately) Anyway, I assume Makita batteries (5Ah or more) can provide 20A current easily. I haven’t tested more, but quality 18650 cells are usually rated 20A-30A. There can be some limitation for electronics inside battery – so 20A is my limit.