I have a box that you could put perhaps three laptops in as a case for this project
OK but
Ideally it would be about the size of a macbook.
We're still dancing around our handbags here.
As mentioned more than once, unless we know the actual specifications of the device we're just throwing idea and suggestion into the pot to no reasonable conclusion.
What you MUST do is have a baseline starting point in order for us to give any answer that is more than just 'speculation'.
As already mentioned there are numerous +12/+5 and +3.3V power supplies on the market already that will work from a DC input (SMPS buck and/or boost circuits). There are also numerous chemistry-specific battery charging circuits on the market - to match the batteries they are meant for i.e Li-Po as a particularly energy-dense version.
Tot up your total power consumption in watts (worst case), multiply it by the length of time (hours) you want the device to run for, divide that into the watt-hours available from whatever battery chemistry you feel is applicable and gather together the parts necessary to built it all.
Don't forget to add in some (reasonable) figure for efficiencies/losses and Bob's your mother's brother.
