I've been making slipcases based on the same basic pattern for over 20 years. It was taken and adapted from a sock case in a book called Creative Gift Packaging by Yoko Kondo, Ondorisha Publishers, Japan, 1986. I always use Canson Card which is 300 gsm and quite heavy.
These slipcases are great for a neat presentation - but they won't withstand heavy duty knocking around, and for that you're always going to need to make a proper sturdy heavy bookbinding board slipcase or clamshell box. But they are very simple to make, light to post, and if a book comes back from an exhibition and the box has been crushed or mishandled, it is quick and easy to make a replacement for it.
In my basic pattern: A = width of spine, B = width of book, C = height of book. I add about 2-3mm. to the measurement of height and width of my book to allow for ease of entry and removal. Dotted lines indicate the lines you cut and solid lines you fold, after scoring with a bone folder.
You can make a wider based open box by substituting the width of the book you want to lie flat for the spine area measurement, but in this case the other sections marked A would be the same height as the sides of your box (which is now the spine measurement). You can then make a second box as a lid to your first box, adding 2-3mm. to the length and width of the lower box so that the lid will slip over smoothly.
After you have scored and folded all the solid lines and cut the swivel tabs, it will be obvious which pieces you glue, with acid-free pva of course.
My next posts will be about variations to this basic pattern and how I made the boxes for That Unbearable Lightness.