The fit is my first hatchback.

now when it comes to hatches, there are points that aren't as stable as in normal non-hatch cars.

which points would bars connecting the opposing side be most beneficial?

roof? underbody? engine bay??

 

The most flex is inside the hatchback shell. Best thing would be to use a rear strut bar or similar, which goes right behind the rear seats, at about the center of the cabin or at least close enough. The front is supported well enough as is because of the firewall and what-not.

 

the hatchback design is like a beer can. imagine grabbing each end or an empty beer can and twisting it. thats the effect on the hatchback design. now take a couple nails and push it through your empty beer can and try twist is. makes it that much harder. so i would focus on getting a rear strut and a c-pillar bar to stiffen up the rear of the fit and that is all you really need unless you plan on turning it into a track car.

 

unless you want to track your car you won''t really need any reinforcements for daily driving, nor will you feel the difference. well, unless you drive your car REALLY hard on public roads.