Then it hit me...
When our conversion was done they left the little flip-over seat behind the driver's seat, which is completely useless and just awkwardly fills the space between the seat and the kitchen unit. By removing that, it left ample space for sub and a couple of sturdy bolt-holes to fix it all to.
I built a box out of 18mm MDF, bought a cheap Chinese amplifier on eBay, and fitted it all together with an old woofer I've had lying around for years.
![Image](http://i.imgur.com/vOPipWi.jpg)
![Image](http://i.imgur.com/VN3ritD.jpg)
When the seat's in its normal position you can barely see it, and you get a nice boom...boom...boom in your kidneys as you drive along!
![Image](http://i.imgur.com/vQLF2Nz.jpg)
Perfect!