Proper modellers do a lot of research and draft a proper plan, with drawings, photos and the like. So far I've just taken a lot of photos of the shed and had a bit of a think. Rather than a static model it would seem more interesting to have some movement of a train in and out of the shed, operated perhaps by an electronic shuttle unit.
And lo, Fosworks (previously Timpdon) have such a unit, which looks just what is needed. It uses reed switches on the train and magnets on the track to control movement.