Above is the whipholder that I created. I modeled it after the sample displayed on IndyGear.com. Here is roughly the measurements that I used - length is 25cm long (includes 1 cm seams, width is 3" + (2) 1cm seams, add a heavy duty interfacing about 2" on each end. Fold right sides together long ways and stitch a 1cm seam leaving an opening large enough to right side out. Lightly press and do a double-row of topstitching. Add snaps on the ends making sure that they will match up evenly. Then stitch the belt holder in place just above the snap. Width will depend on belt using.
Above is the whipholder with the belt inserted and the whip snapped in. It is a little bit tight (I forgot to check this measurement). If I were making this again, I would add 2cm to the length. Otherwise, I think it turned out perfect.