Speaking from personal experience as a Mustang owner, 2 15's is going to be VERY hard to do in an upright or angled box. It will depend upon the mounting depth of the driver and the magnet width. I have a 12" XXX in there right now and it's taking up most of my trunk. However, I'm waiting for 2 15" Avalanches to come in, which I'll be putting in a sealed upfiring box. I don't know about the Mags, but the Avalanches' magnet is too wide to do an angled box without hitting the box.