Whilst searching Makerworld for the exact size of box I needed, there are some great options but they are just too small or too big. Some are parametric, but they only provide a Fusion360 file - which requires more software and faff.

I found this excellent design by Bulbasaur0 on Printables, which is fully customisable using OpenSCAD.  After tweaking the code, it works great with MakerLab (built-in to MakerWorld) to enable you to generate and directly print your own models.  

There is a choice of whether to generate the model by individual parts, such as Top/Bottom/Latch (good for large boxes which won't fit on one plate) and the main options are:

• Width - interior side-to-side size in millimetres
• Length - interior front-to-back size in millimetres
• Bottom Height - interior bottom height in millimetres
• Top Height - interior top height in millimetres
• Corner Radius - interior corner radius in millimetres.
• Edge Chamfer Proportion - proportion of Corner Radius to chamfer outer top and bottom edges. 

It also comes with some fantastic features you can enable/disable, such as latch and seal type, handles and a slide-in label on the front.

There are some advanced configurations for fine size adjustments too.  Once you can download the model into the slicer, you might need split objects across multiple plates for printing.

Printing

Print the box top and bottom parts on their outer faces. Latches print on their side. No supports are needed. For the latches, a brim may be helpful for bed adhesion.  For stronger boxes, latches, and handle, increase to 3 perimeters and 30% infill.

Assembly

The hinges and latches are attached using M3 screws, M3x40 by default. Each hinge needs 1 screw and each latch needs 2. A box with one latch needs 3 screws total, while a box with two latches needs 6 screws total. You will also need M3x55 If you enable a handle.

Boxes with the default sizing options can be assembled with M3x40 and M3x55 hex socket head cap screws (DIN 912).