If Sportkilt is your choice I'd not mess with the straps. Spend that $18 on sporran from SK or Stillwater.

By the time you tack on all the upgrade costs at SK you are in the neighborhood of a lot of other kilt makers.

The perk of SK is that you will have the kilt a bit quicker as they are not tailored to your hip size, just your waist and length.

It will be a great knock around starter kilt. Be prepared to go straight into a craving for another one as soon as you order it though