Cleats
 

Is there any guidance on the thickness of cleats for repairs to a split top on a DB . I have some seasoned violin maple rib - its about 1.5 mm thick ....too thin??

The common wood for top cleats is spruce, not maple.

Go to a piano restoration work shop. They throw away sound boards. Just ask them if you can have what they are throwing away. Thats enough old dry spruce to last 3 years.
YOu can make any many cleats as you need from the old spruce sound boards.