width of materialn in 186 horn

It can be done, but you have to account for the missing  3mm in certain places. You should probably go through the whole plan in detail to make sure you don't end up with gaps or wrong size of panels. 

Extra bracing would probably not hurt. In general, this type of horn requires precise and tight construction  if anything comes loose later, it will be hard to find which panel exactly is vibrating, and it might be hard or impossible to reach afterwards. If possible, rebate the horn path panels into the aide panels, and the braces as well .