This pocket clip is designed to be attached with double-sided tape. I use 3M double sided mounting tape and it works great, but results may vary depending on the surface you're mounting to. 

The tape is meant to go on the bottom of the clip except where the clip starts angling down to form a ramp. This lets the little ramp on the clip come down to meet the object, which makes it easier to slide on and off without getting caught (see photos). 

I printed it on its side with a brim with good results. ABS, 0.20mm layer height.