hi

so i have been thinking about making a new undertray for the engine bay for my car and adding a bit of diy aero later on

so we will start with just the under tray its high heat and impact as the roads are a bit crap and we have a lot of speed bumps

should i be useing fiber glass or kevlar type metreal or a mix?

should i make a mold or just use the orignal part ?

What car are you working on?
The undertray shouldn't get very hot, unless very close to the exhaust. If it is then protect the area with heat shield material.
Unless you plan to hit speed bumps heavily then carbon will be fine. Otherwise either use a carbon/aramid mix or carbon with the bottom layer of aramid. You could also make it entirely out of aramid but it will be less stiff so additional supports may be required.
Unless weight is a serious consideration, fibreglass will work fine and will resist impact better than carbon. It is also a lot cheaper :-)
My undertray is made from 2 layers of 375gm prepreg plus diagonal stiffening ribs. The boxed area is to contain any drips whilst drag racing.