3D Modeling a design is always better than designing something on the fly. Not only is it cheaper, but it’s also faster to have a complete design ready before you start building. Additionally, if you have a model of your design, we can better assist you in building it.
Autodesk Autocad (2D) and Inventor (3D) are good quality design/modeling tools that are free for academics. http://students.autodesk.com/?nd=download_center
Onshape is a Cloud-Based (3D) system that works with any modern web browser irrespective of computer OS. https://www.onshape.com/edu
A few tips for 3D modeling
A paper sketch or something to work off can be useful.
Understand the size of what you’re modeling.
Take into account machining tolerance when designing.