My parents have a large veggie garden in their paddock. They contend with rabbits, Roos, wombats, cows and a horse. They raised the beds with sleepers like you plan to, two sleepers in height. Over the top, they have plastic rods that curl over and provide a frame so an adult can stand up in the garden and then they have a bird net over the top of the entire thing. My stepdad went to the local grain store for suggestions and so far it's worked really well. The net isn't held down tight in some spots so the wind can blow it a little - it's enough movement to spook Roos and the livestock away.

I've also seen a raised bed, again with sleepers two high and all that was across the top was a piece of string with old CDs swinging on it. The birds hated the reflection coming off, the rabbits couldn't get in and the Roos weren't interested.