I've been wanting to do this for a while ever since I saw a company selling them in a very old edition of top gear. After discovering a nice cheap Cobra clubman car seat on ebay I was committed. The cost of the official ones are £200 I could do it far cheaper.
http://www.cobraseats.com/clubman.htm
I took my existing chair which was a leather office chair from argos, and try to match it up with the base on the Cobra Clubman. Needless to say they didn't match. I thought about finding some steel plate, but in the size I needed I couldn't find anything suitable.
In the end I discovered a huge pieces of mdf going for a £1 in B&Q so I thought why not try that.
It took me around 2hrs to trim it down to size and drill all the bolts, as well as cutting down some oversized bolts that I needed.
Anyway onto the pics:
Excuse the background that wall needs painting hehe.
The mdf shows a little bit and when I find half a day I will paint it black to match the chair. Other than that I'm very happy with it. The chair is very comfortable and does all the reclining and spinning that I'm used to.
Few more pics in my gallery: http://www.gordyhand.co.uk/gallery/v.../others/chari/
Hope you like it.