The standard Toyo 4x5 bellows frame measures 158mm square (outside) and 125mm square (inside). The corners are rounded with a radius of about 8mm, and there is a light-trap at the edge - a lip about 1mm thick, followed by a trough of about 4mm wide by 4mm deep. Around the outside, there is a 5mm lip between the edge of the frame and the outside surface of the bellows. That lip is used by the securing slides on the standards, top and bottom.
Note that the design of the Toyo bellows frame matches the dimensions of their 158mm lensboards and the attachment side of their backs. Thus, it would be possible to use that design as the basis for various other special features.