Hi jki, re: extrude - there have been changes in extrude for v3 so that it uses the "asynchronous" type of calculation - maybe that has some bad side effect in your case, I'll take a look at that in a bit and see if I can give you some more information. In v2 the extrusion calculation happened directly inside of the main MoI process, which can be more efficient for simple things but tends to be problematic for complex things that may take some time because the main UI will appear to be locked up while the calculation is happening.

re: second question: you can create a polyhedron using the method described here:

http://moi3d.com/forum/index.php?webtag=MOI&msg=2140.46
http://moi3d.com/forum/index.php?webtag=MOI&msg=2153.1
http://moi3d.com/forum/index.php?webtag=MOI&msg=3798.1
http://moi3d.com/forum/index.php?webtag=MOI&msg=2275.1

There's also a collection of various polyhedra models in 3DM format here:
http://www.rhino3d.nl/pythposter/pyth3dm-eng.html


But another way that might work for you is to use the "Avoid smaller than" meshing option at export time while generating the STL file - if you put in a distance for "Avoid smaller than" anything polygons that are smaller than that size will be broken down by a much rougher angle than the standard "angle" setting. So putting in something that's about say half the size of one of those rivets for "Avoid smaller than" should make the rivets get pretty few polygons for them while keeping other areas with more refinement.

- Michael