Hello all, 

I am interested in designing my own car and about 3 weeks ago, I decided to start learning basic designs on Fusion 360 simply because it cost zero dollars. Given that I had never designed anything before using any computer design programs, I would like you to rate for me this car that I designed. Do not feel bad pointing mistakes because that is how I learn the best. Your genuine criticisms are highly encouraged. 

there isn't really any method, to achieve car panel sized plugs from scratch, that is cheaper that wooden framing filled with foam.
sure this could easily cost some thousands, but we are talking about a car. building one yourself usually means lifetime objective. if you are not willing to spend around 2500€ (wich would be really, really cheap) for plugs and materials for the panels, I would recommend not do it. and 2500 is my estimation of the cheapest possible way an experienced worker could do it, without buying tools, using carbon or sandwich cores, or even advanced processes.
if you want to learn something about wood/foam plug making, search yt for "easycomposites air box". just use some wood panels, instead of the harder foam, they are using.

A lot of metal shapers use was scott was talking about with the wooden sections to help with shaping metal. They call it a buck. It was used back when coach builders were the main car builders until production plants took over. Google search automotive buck or car buck. Another place to learn about this process but won’t be able help with composites is a site called allmetalshaping.com there is a section on the forum for bucks.