A screengrab of FreeCAD showing a red PCB (exported from Kicad, so missing a few components but has the important bits for this design work) sat on a folded cardboard (though it's not totally clear from the render that that's the case) insert with a green rectangular box (which will also be cardboard in reality, and so not green, but it makes it easier to differentiate it) sandwiched in between. The cardboard insert is rectangular with flaps folded down at either side, providing a void below it for a saddlebag (not pictured, that's far too tricky for me to bother drawing up in CAD!); the (green) battery box sits on top of that; and then a flap at the front of the insert folds up and round to wrap the battery box and provide a platform for the red PCB to sit on.