I designed these for 10 series aluminum extrusions that are 1" x 1" square.

key differences over others of the same, the nut is twisted to grab mor metal when tightened, and the corners are rounded for easier printing.

Long version:
As most know t-nuts are expensive $9 US for 25 and then $15 shipping which is outrageous so I used M5 nuts and bolts, I probably should have tried M6 but I couldn't find any at the hardware store and didn't want to buy 200 of something that I had to grind the edges off, all I could get at the hardware stores was 1/4" (grind it), M8 (way to big) and #8 screws (too small) so I went with something that I knew would grab but didn't need to be ground but they are too small to touch the sides and so would spin and would flop around in the channel so I printed some holders for the nuts, there were quite a few out there already but they all have the flats of the nuts parallel to the flats of the channel and they don't overlap that much so I could see over tightening the screws and it pulling out of the channel so I made some that the nut is turned so that it grabs as much of the channel as possible, I also rounded the edges for several reasons:

1. Printers move faster along curves than they do corners.
2. When printers do corners they can leave extra plastic as it pauses at the corners (on mine anyways)
3. When printing without fans (I was using ABS as I have lots of it) the corners of things angled up REALLY start to angle up and if it comes to a point it will cause a bulge up at every corner that needs to be cut off.
4. Less plastic made them print faster.

Update 8/27/2015:
I needed some holders with the nut closer to the edge to get in close to the corner of a printer I am making, I shortened just one side of the holder and included it as well and it is also in the sketchup file.