Personally I’d love to be able to go with the simplicity of the straight across 9-3 cut. However, to ignore the knowledge of the ancient egyptians risks a mouldy lunch, as they proved that the shape of a pyramid preserves whatever is onside (you will all be familiar of the story of French explorer Bovis finding the preserved cat in the Great Pyramid).
For that reason I’d argue that triangles in the exact proportion of the pyramids is the only way to go. An equilateral triangle with a base of 440 units and sides of 418 (see diagram below) is the usual proportion chosen by sandwich makers with an eye on the longevity of their filling, such that the vertical is therefore 356 units.
You’ll be pleased to know that use of the Egyptian cubit as a measure is not required, as that would make for an unwieldy sandwich. It’s just the ratio that matters.
You need to adjust the bread by trimming unless you make a custom tin for home made loaves.
I’ve attached a diagram below.