A bee box-making jig

As I had a number of brood boxes to make up, today I decided to make a jig that holds them square and aligns the frame rails at the correct height. Only when it came to making the parts for that last job, I discovered that the supplied boxes and parts weren’t the same as the published dimensions. I’m guessing that’s partly down to the original boxes being specified in inches whereas everything is now done in millimetres and people sometimes round up and sometimes round down when converting between the two. I think I need to do a bit more research before comitting myself.