Floor Pan
It is my suggestion that the floor pan now be installed onto the kart. I recommend using "Ribbed aluminium Checker Board" (3-4mm) for the floor pan. If not 1mm mild steel sheeting will have to be used. This can easily be shaped with a "jig saw" with a steel cutting blade. The checker board can then be shaped to fit the curve nicely. If you try to use an angle grinder you WILL fail!!!. you can however if you have a few cutting disks to spare you can clean of the burr. Once the required shape is cut out, it is just a case of fixing it to the chassis.
You can just bolt the whole pan to the chassis by drilling a hole through the chassis and the pan, and fixing it on with a 5mm bolt and rubber grommet/washer. However this severely weakens the chassis, and a better option would be to weld pieces/ tags of metal to the chassis and to bolt the pan to these tags instead. Only a few are needed. It depends on the strength of the tags.
Mount the floor pan on top of the chassis. Also it would be an idea to keep some of the "aluminium Checker Board" for later, as it will be useful for the pedal floor pad, for one's feet to rest on.
