I use plastic strip, rod and card to make these these and sizes are normally just judged by what look rights. Crates and boxes come in a variety of shapes and sizes but these are mainly based for ww2 onwards.
Far left plain hollow bar, centre as being capped at one end with thin plastic card, right side plastic card to stick on to the far end of the hollow bar. This creates the solid box shape for the crate.
Picture 3, bottom hollow bar shows area to be cut out to create an open crate, top picture shows the area removed and cut to size.
Picture 4, shows one end capped and the other piece of plastic card ready to cap the other end.
Picture 5, cut short lengths of thin bar to stick on the end both open and closed crate.
Picture 6, thin bar stuck to the end of the crates.
Picture 7, cut and glue a second bar to the opposite end.
Picture 8, thin bar stuck to both ends on both open and closed crates.
Picture 9, cut two strips of plastic bar for the bottom of each crate.
Picture 10, the two strips stuck in place on both the crates.
Picture 11, the solid crate now as the two strips of bar stuck on the opposite side.
Picture 12, shows the lid cut out of plastic card and the two strips of bar cut to stick on the outside of the lid.
Picture 13, the thin strips of bar stuck in place.
Picture 14, the open and closed crate completed.
Various crates and boxes made using the above method, adding different bits and pieces of thin plastic card and bar.
Hopefully I've explained it well enough, and the pictures give you a rough idea to go by.
So thanks for looking and hopefully it's of some use.
Cheers for now Russ