This is the counter-rotating version of my 2.5in 3 blade spinner (http://www.thingiverse.com/thing:44872). The negative offset base is unchanged from the normal rotating spinner. I uploaded it again for convenience. It is designed for a 8" or 9" x 6-7" 3 bladed counter rotating prop.Version 1 of the base has an negative offset, meaning the bottom edge of the spinner will be "below" the prop adapter by .3125". Version 2 has a normal offset of about .050".The spinner cap is the same for both.The cap and base are screwed together by M3x12-14mm socket head cap screws.This should be pretty light, but I haven't had a chance to actually print this one out.