This is a remix of http://www.thingiverse.com/thing:145567
I have changed it a bit to fit my muddyfox bag.
The clip will hold a 10mm pipe and the strap slot is 32mm long and 2mm wide,with the return on the end it can be slid over the strap and is very unlikely to come off.
The clip is 10mm wide and I printed mine in PLA and has a good amount of flex in it to get over the strap.
I printed mine at 0.2 layer height and 40% fill in white PLA and it came out nice and has enough flex to get over a thicker strap.
Print and get running :)