Hi Dan
Your posting made me race down stairs for my C5's boot & immediately started to try and squeeze my older style 17" PowerBook G4 into the boot.
Due to it's narrower opening I found my laptop had to go in diagonally & would then sit in the boot comfortably, So was it Johny's comment about it being larger on the inside than it first appears.......
However the lap-top might fit, but when in my waterproof throw down stairs proof lap top bag it won't........So I will be interested to see how your design mind makes your machine safe from water and road side shocks and bumps.
Wonder how you've been getting on?
Neil