There is indeed a magazine component you can design, it's somewhere near missile launchers on the component design window. You choose the size if the magazine and how much armour to give it, if I recall correctly your first magazines should hold around 15 MSP per 50 tons devoted to it. As you research improved magazine feed system you get increased capacity per 50 ton increment. You can also research improved magazine ejection system, which reduces the chance of a fatal hit to the magazine causing a massive secondary explosion.