Upgrade mid 2011 27" iMac: replace Superdrive with SSD and create Fusion drive?
Be carefull when working on an imac and ensure you have the correct tools - corrct size Torx drivers and a suction cup or two to remove the glass safely. Also use iFixit for a guide on how to do this such as https://www.ifixit.com/Guide/iMac+Intel+27-Inch+EMC+2309+and+2374+Optical+Drive+Replacement/1637

I would suggest a superdrive HDD enclosure. You will need a 12.7mm one such as: http://www.amazon.co.uk/adapter-caddy-Apple-iMac-slot-/dp/B00C0NU1TU/ref=sr_1_1?ie=UTF8&amp;qid=1415181905&amp;sr=8-1&amp;keywords=imac+superdrive+enclosure They are around £10-£25.

Ensure that you remove the optical drive thermal sensor and replace it back onto the SSD.

As far as SSD choice I have used Crucial, Samsung and OCZ. The Samsung evo series are very good speed over price.

I would also suggest moving your mac os install to the ssd and using the normal HDD as storage, this will improve speed.