I would like a comfy back carrier for DS (7 months but big - 20lbs and in 9-12 month clothes, length-wise). I have been using a wilkinet but only in the front carrying position - never mastered the back carrying one - and he is now too heavy to be carried on the front. I would like something he'll be comfy in for long walks on the moors, and that won't ruin my back! Recommendations please?