As much as you may be able to obtain adequate results using them free air you have to consider the effort and cost required to seal off the trunk using deadener and expanding foam vs the results.

For me simple wins, building a couple spec'd sealed boxes will give you the optimal results from the subs. Deadener may not even be required on the deck depending on how it sounds and how it is mounted.

Which ever way you choose I am sure you will get the results you are looking for.