Might I suggest that you take the sporran belt and mark off the area in which it rides normal for you...

...and then take silicone gasket sealer and place a small dot on the back of the sporran, right between the straps...

...and then coat the belt about 1/2" to 3/4" wider than the sporran straps are.

This should give a nice grip between the two and allow you to slide the sporran when you need to.