A couple of things:
If the battery pack will be at the front of the car it makes more sense to run the wiring to the jump start terminal in the engine bay fuse box, as it'll be an easier run
Unless you're going to use one hand to push the 'momentary' switch and the other hand to push the Start button/turn the key, you'll be better off with a 'normal' on/off switch
EDIT: Ignore my first point - just noticed that you have a 400h, not a 450, so the battery is up front, not in the boot.