Now by popular demand: #8 and #10 versions
A simple clip that takes two #6 screws and has a curved cavity through which you can shove a zip tie easily. Most zip ties up to 30kg (70lb) size should work, the zip ties I use are 11" long and rated for 70lb and fit fine.
The cavity is 2mm deep and 6mm wide - in practice you're not going to want to try and shove anything more than 1.5mm thick through it.
I would imagine it's quite strong but I have no means of testing it's strength.
Print it on it's back with no support. If you can't bridge 6mm, you should look here or here to be able to do this cleanly.
I recommend printing at 0.1mm layer height; if you print at 0.2 you will further reduce the vertical clearance in the hole.
Hole-to-hole spacing is 28.1mm (1.1"), and total width is 38.1mm (1-1/2" by design).
#8 Version: 42mm (a tiny bit under 1-5/8") total length
#10 Version: 49.5mm (just a hair under 2") total length