[Arran]

If you could take the cabinet apart so you can deal with the arch seperately it will make things easier. If you can do that then you can use a larger piece of veneer then what you need with the contact cement and then trim the overlapping veneer to size after it's glued on. I believe that they used plain cut French walnut veneer originally but if they used an extra dark walnut finish over top you can use black walnut or just about anything. I don't know about the 16B cathedrals but some models had the grain of the veneer running from back to front or side to side over the arch depending on when the cabinet was made during the production run. Usually when it gets to the point where the cabinet looses the top layer of veneer of the arch the other layers of the arch are just about to fall off along with the rest of the cabinet.
Regards
Arran