I also have the Kel-Tec case which is nice and made for the sub. It has loops for two spare mags, so with a standard length mag in the rifle, a stardard length mag in the upper loop and an extended (30 round) mag in the lower loop, I can carry 60 rounds (50 if you don't have preban mags) of 40S&W in a very compact package under the seat of my truck.
You could carry even more if your sub is a 9mm and you have pre-ban standard capacity mags.
Have fun, it's a neat little gun to shoot.