These sort of things will fit over the top of a 30cm tank without too much over hang.
http://www.trademe.co.nz/pets-animals/fish/lights-lamps/auction-456996165.htm
http://www.trademe.co.nz/pets-animals/fish/lights-lamps/auction-457368032.htm
After having had a clip on light for a year (?) on my small tank and then getting this sort of thing, I can say that I wouldn't go back to clip ons unless you are electrically/DIY minded enough to rig up something that runs the length of the tank with a decent wattage. With a clip on you don't get the light penetration to the bottom and to the corners so the only stuff that grows OK is very low light or stuff directly under the light. Clip on LED may work better than clip on fluoro type tubes but I found a solution in the above type of light instead.