Hey, all. I got into doing some 3D designs at Shapeways recently and designed a bunch of add-on parts for Combiner Wars Ultra Magnus. Plus the 3rd Party hands add-on and the single missile launchers I'm ordering from an eBay seller overseas.
The legplates and the waist pieces are what I think really worked out best. The legplates work perfectly in all modes. The waist pieces have to be removed for certain poses or to transform but really shift the feel of the figure. I designed flaps to go with them but you'd need to print in HD for the nubs to stay attached, unfortunately.