One other thing you can do is a create a new page not linked to your navigation menu (as mentioned above), but create it as your under construction page using HTML. Temporarily set that as your homepage. From there you can deactivate your menus (or not) and do what you need to do regarding design and construction on all of your pages without manually deactivating each one. When you're finished, set your original homepage back as such and you should be good to go. I hope this helps.