Apples... but which form?

Morning All,
i have almost finished building the veg garden area and half way through constructing the perimeter fence. It’s 70cm high and 3.5m wide in this section and I was thinking of training an apple into it by forming a planting bed in front of the sleeper. What form would be best? I thought a step over, but would I have enough height for a double step over? I would love an e a palier but not enough height here I think. How wide would a step over be- it would ease my need for symmetry that it would fill a good part of that width if not all eventually- is this dependent on rootstock? What do you think? It’s west facing so gets a good amount of sun- soil is awful but would dig out and improve.