The tops on my flattop mandolins are 3mm thick. I use European or Engelmenn Spruce, or King Billy Pine tops.They are X braced with carbon fibre reinforcement and are radiused with a 15ft radius. Bracing is more important than thickness because it is the braces that provide just about all the structural strength. Just an X is not sufficient, mine are braced similar to a guitar, but without the tonebars and with additional bracing under the tailpiece tp stop the top folding in under the tailpiece. The X and the cross brace are carbon fibre reinforced. There is no bridge patch. Flattop mandolins have a nasty tendancy to fold in around the soundhole, so the challenge is to have the thing strong enough so it won't fold up, but at the same time to be flexible enough to sound good. Experience has shown me my system works. The mandolins are stable, they stay in tune, there is hardly any sinkage in the top when the strings are installed, and they sound great. Not as refined sounding as my carved top mandolins, but are noticably louder and with huge ring and sustain, and most important sound clean, unlike a lot of other flattop mandolins. You can play them real loud with no loss of sound quality. People seem to like them. They are fun to make and play.

Do you have a pic of your bracing?