Given your time restriction, a Rob Roy is probably the best bet. Even the basic 'regimental-type' day sporran is going to take more work than a beginning leather worker will be able to accomplish in 4 hours.

I have two suggestions for you;

1) look for (or develop) a 'gathered' type Rob Roy pattern which utilizes a circular ONE-piece main bag, NOT the more contemporary 'freelander' style Rob Roy (which requires a LOT more sewing).

2) make sure to offer the pattern with the option of either the sporran strap back or a waist belt mount. This will allow you to advertise the project as SCA/Renfaire/pirate as well as Highland. This will probably double or triple the number of people interested in the class.

Make sure to update us on the class (and post pix)ith: