If this model strictly 3d printable, there is no problem with it. If it's gonne be made other way, model is pretty bland, and will require further technological processing.
You didn't put any technical tasks here, but it subjectively looks a bit bulky for me. The walls look fancy, but very possible it will be not very comfortable to work with any load straps.
Another thing that discomforts me - the bracing. I think you will not be able to put on the roof of any car, without additional support without damaging the roof, even if it has some answering holes. You need to make roof specially designed for this kind of carrier, or additional support bracket (and pretty huge, taking into account overall load capacity). I would not pay attention for it, but you have assembly of carrier with a futuristic car (Hi, Elon)

I know the only way to improve - take challenges, and make projects to the very end - materialy existing and working.