Den 290mm is quite a big wheel. Most prefer a smaller 260mm or similar. The smaller wheels have 3 bolts attaching them. If you simply want to change to a smaller fixed wheel you will need one of these (and the wheel of course). You may also need to re wire your horn to a button on the dash as I suspect your existing wheel may have a horn push in the middle. If you do want a quick release wheel then you will need a quick release fitting and suitable upper column. Those suitable for a 7 are drilled for 3 bolt attachment, but it is possible to use your existing wheel if you purchase (or make) an adaptor plate, but I think you will want a 3 bolt small wheel. As for quick release attachments, there are many, but only a few suitable for a 7, as some are too bulky and also push the steering wheel much closer to the driver. Those fitted by Caterham are Racetech or Lifeline similar to this, but as has already been mentioned, these tend to wear after a while and become sloppy. The Rapfix is highly recommended and doesen't seem to wear, is very easy to attach, but is more expensive.
If I were buying again I would opt for the Rapfix, although it too pushes the steering wheel a little closer to the driver, but only an inch or so. Hope this makes sense to you. Good luck.