Most of the angle I use is 3" x 3" x 1/4" 6082 but you can get 2" x 2" x 1/4" as well as many other sizes.
I get mine from Richard Austin Alloys but any supplier of Aluminium will likely have it.
Obviously with you only wanting 300mm then it would likely have to be metal supermarkets, eBay or such like as the metal suppliers sell in 5 or 6m lengths.
Afraid I don't have any 2" x 2" or I could have sent you a bit.