I know there are other things for M8 or 8mm Trapezoid leadscrews on Thingiverse. But I couldn't find any delrin or other nuts to buy, the only ones i could get are 17mm wide.
So this is my Design for these nuts. I kept all original i3 distances, so you might have to file down one edge of the nuts to fit next to the LM8UU bearings.
Also, I added a belt tensioner. The x-idler prints with a small support there, just break away after printing. Use a 623 (3x10x4mm) bearing.
Just in case you are wondering why the picture doesn't look like an i3, I gave mine a major frame upgrade. Take a look: http://www.thingiverse.com/thing:756841
Update 25 Apr '15: Added more space to insert nuts
Update 24 Okt '15: Uploaded belt guide file