Thanks to everyone for their advice! I decided that building anything was beyond me (lack of tools, space, time and ability), so I decided to see what I could find in the area.

Well, I went into my local car audio dealers yesterday (the ones who fitted my Empeg, amp and front speakers), and after a bit of a chat and a listen to some of their subs, I got what I think is quite a nice sub in a pre-build sealed enclosure. I've sacrificed my rear (stock) speakers, and wired the sub into the spare channel on the amp.

It's certainly a huge improvement in sound quality, and since it's running off my existing amp it was a two-minute job to fit (and even quicker to remove when necessary).

Actually, speaking of the car audio dealer (Berkshire Car Sounds in Reading, FWIW), I noticed that they have an Empeg sticker in their door window...

Bryan.

Mark 2 #080000515
12GB Green (was Blue)
...but I've only got 2GB left to fill!