I haven't read the whole thread so I don't know if this has been mentioned before but I use a vernier height gauge and an angle plate for marking out boxes. In industry they are used on accurate surface plates but for sheet metal any flat surface (the kitchen worktop) will do.
The height gauge will scribe a line at a given height, so it is easy to scribe lines parallel to an edge or side of the box. Use a marker pen or engineer's blue to colour the area to be marked first so the scribed lines show up better. To get centre lines you can use the gauge to measure the box and then half it. You can easily check a center by flipping the box over and checking that the gauge meets the scribed line.
It is quick and accurate and works with cases rounded edges and corners and domed tops.
The height gauge will scribe a line at a given height, so it is easy to scribe lines parallel to an edge or side of the box. Use a marker pen or engineer's blue to colour the area to be marked first so the scribed lines show up better. To get centre lines you can use the gauge to measure the box and then half it. You can easily check a center by flipping the box over and checking that the gauge meets the scribed line.
It is quick and accurate and works with cases rounded edges and corners and domed tops.