Thankfully the process is simple using AC3D to model and then export to second life. We had a similar script running from SketchUp but as they both only import boxes we assumed you were better off building direct (perhaps we were wrong as the movie below illustrates the potential):
The AC3D Second Life box export plugin works by generating a second life box prim for each object in the AC3D world. You can either make boxes in AC3D, or if you just want flat rectangles, you can simply make AC3D rectangles. You can use whatever shapes you like in AC3D e.g. spheres etc. but everything will appear in SL as boxes.
Take a look at AC3D for more details and Happy Boxing Day :)