Public Lab has developed multiple open source Do-It-Yourself spectrometer designs. Choose between the following:
USB Desktop Spectrometry Kit
a precise and inexpensive webcam-based design built in a conduit box
rigid plastic spectrometer attachment for your smartphone -- robust and very high precision
All of Public Lab's kits are open source - you can always make your own using the listed parts. The store and the Kits Initiative are intended to help community members cheaply and easily get started.