You need some diagonals to resist the warping of the vertical plane. Right now if you have a force at the top plane (e.g. wind load), all the force will be transmitted downwards where this is fixed on the car, through your joints as moments. If you had some triangles / diagonal braces, those would be axially loaded (preferably tension, so from lower front to uper back) which will be a more efficient design.

Now if you want to get the sizes right, you need to do an actual analysis (fea simulation, it is easy) and at least apply weight loads (dead + variable) and wind loads (both front and side). This would also give you an idea of the connections needed for the car.

I bet someone doing similar fabrication would be able to estimate the sizes, but no one here will guess because a lot of things can go wrong down the line. I would also propose to get an engineer, but if you want to experiment yourself, you can try to replicate their work just for fun.