I don’t think there is much point in making the door stronger than the frame/walls so 9mm sheet sounds good.
All you can really do is make it time consuming for someone to break into.
Yep, time consuming and noisy …
I just used Tee Hinges with a few coach bolts, like this:
I’d say your biggest issue is going to be the lock, i can’t see you have any other option than a hasp/padlock but make sure you use dome bolts not screws with a backing plate (something like 3mm aluminium). I’d do something similar with the hinges.
Ah, good points don’t kinow why I hadn’t thought of coach bolts…
I’d thought about some type internal hinge but like you say, no point going overboard .. the OSB on the walls is by far the biggest barrier … but it was a garden shed rather than bike shed when conceived…