Technicaly speaking, the frequencies subs put out are unidirectional. However, that's not entierly the case(in a perfect environment, that would be totally true). Sub bass frequencies do bounce like mad off of almost any hard surface. The way your sub sounds depends on a few factors. Enclosure type, enclosure volume, sub type, and a few other factors I probably forgot to mention. Also, every brand and model of sub have different sound qualities.
Bearing this in mind, here's my suggestion.
Try either a homebuilt box or buy one. Just make sure you looked at the spec sheet for the sub and give it the propper volume and enclosure type for whatever sub it is. (make sure it'll fit in your trunk) Then, just experiment. It'll take a bit to figure out what works best. Everybody has different expectations as to what their subs should sound like, so it's pretty much a judgement call.
I find that my HE2 10" (fosgate) does best in a rear corner of the trunk facing upwards in a .75 ft3 sealed box.
It's all going to come down to what you like.
Hope this helps!