You know mounting subs inverted is really only for looks. You're not saving any space since what you took "out of the box" is now jutting out into your trunk, it'* magnetic as hell, and you could damage the woofer if you're hauling around a bunch of stuff in your trunk.
Go out and take some measurement of your trunk - maximum width between the wheel wells, maximum height under the frame bracing, maximum depth you think you want the box to stick out, and the angle of the back seat so it'll sit nice and flush with no wasted space. I wouldn't think mounting a 15 vertical in the trunk would give you clearance problems, but you can always angle the front panel to fit if needed.
Then, get some basic info on your woofer. If you don't have the original material, go online and look for a spec sheet. There'* no sense in building a new box if it'* just going to sound like something farted in your trunk.
Then go here: www.bcae1.com
. Click on Topic 110 on the right side. It'* a box calculator, and you can tweak it real-time. It will save you time and money, and lots of it. It'll even give you a template sheet for cutting your wood. It'* pimp. I came across this site one day and spent 6 hours reading all the topics - it is everything you ever wanted to know about car stereo...
Anyway, unless you have a beast of a 15 that takes mucho cubic feet to run right, you'll be able to build a simple box that fits in your trunk the way you want it to, you won't have to have your woofer sticking it'* butt out at everyone (unless it'* all chromed out and pretty like harofreak'* and you want to show it off). It'll sound great to boot.
Good luck bro