This instructable takes a huge chunk of inspiration from a pre existing lightweight rear mudguard that is already commercially available. I have adapted the design slightly and, in addition, created a small guard for the front wheel.
These mudguards are small enough to keep in the bottom of your bag for whenever you may need them AND this whole instructable cost me only £1!!!
Step 1: Find your material!
You need to find a suitable plastic material with which to build your mudguard. I considered using the thin plastic sheeting that document folders are constructed from however i decided it could end up being a little flimsy. I found this multi pack of flexible kitchen cutting boards in a super market for £3. The material that they are made from is flexible, strong and light. A single cutting board will be enough to make both front and rear mudguards.