The bandsaw cuts to the scribed marks.

The corresponding cut at 90 degrees can either be done by hand,

...but I prefer to make the cut with the bandsaw. A square block held on my cross-cut sled (which runs in the mitre slot) holds the workpiece at right-angles to the blade and the piece is gently moved into the blade until the piece falls out. A tad more dangerous than the tenon saw method - be warned!

This is the dry fitted corner butt joint.

Next we need to set up a 1/4 inch router cutter to cut a slot to hold the top and bottom of the box. Set to 4mm height approx.

Then using a 4mm drill shank set the router 4mm from the fence.