Thinking of the hook for the wheel and a separate part to attach bike to with D lock. has to be this way because it's all the space will allow unfortunately, so that rules out the horizontal mount idea.
i think the brighton bike sheds vertical sheds work similar to how you've described.
there's a hook for the wheel, then a scaffold pole built into the frame about half way up to so you can put a lock around the pole and the BB area
Thinking of the hook for the wheel and a separate part to attach bike to with D lock. has to be this way because it's all the space will allow unfortunately, so that rules out the horizontal mount idea.