This is a dummy/insert battery for my Canon EOS 750D that uses a LP-E17 battery. It can be used to power the camera with mains electricity or from a battery (12V lead-acid in my case) by using the XL4015 DC-DC converter. Follow the specs of the converter for the input and set the output at around 7.2V.

The box for the converter was designed by McDoS: https://www.thingiverse.com/thing:2808246

As you can see I had some filament issues so the print is not a beauty but it works fine.

I measured the dimensions and copied the shape, but making it hollow to fit wires. The battery has a +, -, Thermistor & Data pin connection. Only + and - need to be connected to 7.2V. I did this by scissoring some alluminium strips from some scrap I had laying around and mounted it on the correct pinholes, but there are many more ways you can come with to do it.

The camera then asks whether it is a canon battery: choose no. Then it asks whether you want to continue using the battery and then press yes. The camera then works completely fine even though it indicates the battery is empty.

Feel free to ask questions in the comments!