A racing seat is just not a good looking bucket, it is designed and built with safety standards which are internationally recognised and followed. The primary purpose of a proper racing seat is to keep the driver harnessed to the seat in case of an impact and to be structurally strong so that it does not break apart. Special shoulder harness type belts are also built into such seats.
What you are designing from this sheet of Tin or aluminium is very dangerous. You have no means of calculating the strength of the metal, it's load characteristics, the strength of its welds or joints, or any other technical parameter. You will also not be able to perfectly install the safety harness, and therefore, you will be literally exposing yourself to the most extreme risk just because of this seat. The roll cage, in case of a severe impact to the car, god forbid, will act as a solid metal container in which the driver will be hurled and smashed around if the seat itself breaks at any of its weak points.
My advice to you is: please don't save money on safety. Get a proper racing bucket. If a high-end professional one is too expensive from America, just get a used Japanese racing bucket from some salvage yard, you can find some really good ones there at times. It might be a few pounds heavier, but it will certainly carry your weight in the worst of circumstances.
