Well I don't own a real supertuck but my DIY version of something like it I just took some 800 grit sandpaper to the back of it. I also worked with some stronger paper but I felt 800 worked fine, I probably would have used 200-400 but I didn't have any handy, I just had 60 or 800+

Either way as long as you don't take too much material off I would just use a little sandpaper but it's your holster. The other thing I've noticed is that most people seem to wear an undershirt between the holster and body with supertucks, I assume your not and it's rubbing your skin.
So either way that's what I did to mine, and what I would recommend but someone else might have some other idea.
Luke