We have several raised beds made out of oak sleepers. They are not cheap - but they are easy to make, and are high enough to prevent the carrot fly from getting in there.

Basically, each long side is made up of 2 sleepers, and the ends are two half sleepers on top of each other. Therefore it takes 6 sleepers to create a very substantial raised bed. They're screwed together, via brackets, on the inside and then filled with good quality topsoil. [it takes rather alot of soil, so this can be a hidden cost which people dont' think about.]