Tools for the job - plastic case cutter
For a long I used a small hacksaw to separate the two halves of boxes, there are two disadvantages with this method one is the danger of cutting the contents another is the wide cut makes it difficult to re-seal the box.
The cutter in my flexible drive.
The blade for the cutter is made from the end of a hacksaw blade, brazed onto the end of a rod that was the correct size for the collet on the flexible drive.
Once attached to the rod the irregularly shaped piece of hacksaw blade was rounded on a bench grinder and ground to a blade profile.
The teeth were then cut using a dremel style grinding disk.
A sleeve is used to set the depth of cut, in this case a small sleeve gives a deep cut.
A large sleeve a shallow cut.
I have used this tool dozens of times to open battery packs and power bricks, some care has to be taken to keep the cutter dead straight and of course like any sharp tool away from flesh. The cut using this tool is narrow enough to neatly re-glue the box after repairs/modifications have been made to the contents.