First: I would make up a practice template in ordinary paper to to make sure I have the dimensions right, before committing to card.
The box should have a base, just a little bigger than the 'card'. Off each side of the base add a rectangle that goes just a little higher than the 'card'. Add glue tabs to the side of these projections. One per joint should do, but may wish to add extra tabs to the template so you can decide which ones you need. A lid can be created in the same way, just expand the 'base' by a couple of mm so it will fit over the main box.
If you work with a paper template you may also discover how to make an attaching lid - I tried to add a photo of one I made to take some silk squares -
http://angelsilk.co.uk/gallery/squaresbox500.jpg
Hope this helps.