Interesting photo but why cut into the roof so far forward, just keep hatch open and use one of these? Habitents Prius Tent For Hatchback Car Camping - Gallery https://www.autoanything.com/truck-tents/toyota/prius/111A50603A50606A0A17A120A1.aspx You Can Buy This Tent For A Prius That Fits In The Glove Box
Hatch was open but look at how far forward the roof is open. Or maybe it just looks that way from the photo. At any rate it looks like there are many alternatives including those I cited.
Amusing idea. Unfortunately it pretty much converts the car into a convertible. This car's design must have included the extra stiffness from having a full cabin, that is, a stiff boxlike structure between the wheels. Cutting the top off would likely let this body flex much more than was intended, which would not be a good thing.
What components would be to be modified to stabilize do you think? Just the bottom frame not the upper / roof
