If you want very deep beds it might be worth considering a Solar Tunnel – they have straight sides so there would be good access right to the edges of your tunnel. The other advantage is that the polythene isn’t buried as the frame is attached to ground anchors, this means roots have access outside the tunnel around the edge which would save on watering. Personally I love them – somewhere nice to sit with a cup of tea when it’s raining! They really add to the choice of crops you can grow and extend the season by weeks either end.