This is the 3D CAD file that I used to create the new conning tower for the upgrade project I conducted on my Thundertiger Neptune RC submarine recently. You can view a detailed overview of what I did and how I did it in this video. 
 
This STL file allows you to print off your own tower. You'll need to add the holddown hardware (I used magnets in my build, but you can use bolts, tabs or whatever method you'd like). The big "N" crest was a physical cast part. If you'd like to add that to your model as well, you can purchase it here. It will need to be heated up and conformed to the contour of the tower's front face.