Hi Stephan,

Not sure how wedded to box2d you are, but there are many ways to do this, but the first function you will need, is whether a point (or set of vertices) are inside or outside of the the desired polygon. You should have a look at Paul Bourke's amazing site which documents a number of incredibly handy geometrical methods. He describes a simple routine for testing whether a point is inside a polygon. Written in C so is very easy to read for a java user.

http://local.wasp.uwa.edu.au/~pbourke/geometry/insidepoly/

After that and depending on your approach (static or dynamic) you could simply create a rectangle class (defined by a centroid, width & height) of your own, and then use a while loop to place centroids of a new rectangle, always querying whether the proposed new centroid locaion is sufficiently far away from the existing set of rectangles (ie... dist2other > (myLength/2 + otherLength/2) || dist2other > (myWidth/2 + otherWidth/2) ), or you could go the whole way and use a bit of attractor math to get the rectangles to pack. either way should work.