4th order for old solo 15c (round sub from '99)

[snapper_slapper]

Ok, here's my first post. I recently replaced the HU in my sons Scion and the guy I bought it from threw in a cheapo Pioneer amp for free. The amp went straight to the junk pile in my basement until 3 days ago when I got bored. I also had an old solobaric 15c that I've had for the past 13 years buried in the pile still in the sealed box I built for my Ranger. Plenty of wiring there too, so I hooked it up. I was actually kinda surprised it sounded as good as it did on that amp so I bought a decent 700'ish rms to put on it.

If I leave it in there it'll need a new box and since the old solo's don't like ported, I thought maybe a 4th order? I've got about 4.5-5 ft^ cargo area to play with. Would it be worth it to go 4th over sealed? Or would I be better off just pitching it back in the pile and going with a single L7 10 I also have in a sealed box? This is his first system so I don't want to tie up a bunch of money into it. Either of the two subs I already have will be good enough till he can buy his own shit.