How do you bend aluminium/steel bars?

Use a pipe bending spring inserted in the pipe which will stop it flattening or hire a pipe bending machine from a DIY hire shop. In your place I would make a drawing of your bends and take the tubing to a small workshop or garage where they will do it for you. It is very difficul to make good bends and DIY runs the risk of weakening the metal which could be dangerous in a buggy. OK it will cost you but it's your life on the line.