I'd say that it's how much boot space you're willing to sacrifice. And how much you want to spend.
But, for a 10inch sub... you can probably get away with a small sealed box.
As for making a box. I've made heaps of speaker boxes. It depends on SO many scientific thingies to get it right. Just buy a cheap 10inch box and rid yourself of the financial burden of having to make your own. Most people can't tell the difference in sound anyway.
At Strathfield Car Radio, they've got POWERED subs in boxes, for $88. These are good value!
For more details on making boxes, properly, PM me.