Here is online modeling tool for 3D printing: Three Park. You could think of it as an online Sketchup, but more powerful. You can do modeling online(no need to download or install software or plug-ins), store your designs in the cloud and export STL or OBJ for 3D printing. What’s more, modeling service is free forever.
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Sounds pretty straightforward, look forward to giving it a try 
Cheers
