Most importantly, it's important that the center of gravity is a low as possible. So if you want to put the cargo's weight between two wheels and sit somewhere

Most importantly, it's important that the center of gravity is a low as possible. So if you want to put the cargo's weight between two wheels and sit somewhere in the center; it's better to have a customized standard frame with saddle in the back to make construction as easy as possible. A side effect of the cargo weight in front and the drivers in the back is that the load weight per wheel is more evenly distributed. Also, as mentioned before, it's easier to monitor cargo, kids and your vehicles maximum width in relation to any obstacles in front of you.