If it needs to be lightweight check out the Bill Fitzmaurice Simplexx 15HT sub, it uses lightweight braced plywood construction, corner ports to reduce cone rock and will offer plenty of output for your room size. Its a DIY sub project but there should be a cabinet maker near you.
The weight should be near 1/2 of the others mentioned, it uses the Dayton HO 15" driver
Here is some info.
BFM web pages.
S 15 frequency response