Are you using the Bag-in-Box system to fill up your homemade juice?
Then this is an good alternative to using the normal cardboard boxes.
Its designed to make good use of the material and to be long lasting.
800* 400* 6mm MDF or whatever you want (whats MDF in english?)
Step 1: cutting
Saw the parts as in the picture below to make best use of the material.
The first cut should be on the long side, so then you only need to cut pieces off of the two resulting boards.