Sometimes you don't want everyone to know you are carrying around your iPad. No one will notice that you are carrying a book, especially if it is a 1970s ex-library copy of "New Zealand in Colour." With a hobby knife, a paper clip, and a bit of patience, you will be able to recycle the very thing you are replacing into a case for its replacement. That could have been said simpler but you get the idea.
Step 1: Find the right sized book
Take your iPad to a second-hand book shop or thrift store and find a book that is the right size. I found the best size was when you set the iPad on the pages of the book, there is about 20mm (3/4") to 25mm (1") all around it. Too small is a problem because the paper won't be strong enough to hold the iPad in and too big is fine, if you like carrying alot of size and weight you don't need. The perfect thickness is when the pages of the closed book are the same or slightly thicker than the iPad. I also chose a book with lots of photos because the pages were thicker which meant stronger and less to cut.
On second thought, why not breath new life into that old Masters thesis as a personalized iPad case? You're probably not going to read it again are you?