How high are your ceilings? If you have a utility room you could hang them from the ceiling. Years ago at uni I used to put mine in a survival bag under my bed. It was slightly impractical though. No secure bike storage. It wouldn’t have been that bad except there was a canoe in the middle of the floor and my shower was used as a drying room for my paddling and biking kit.
Personally I’d sort the door on the shed, put in some anchors and get an alarm.
There are metal sheds you can buy that you can bolt to the floor and lock. I know someone who assembled one inside the shed rather than go to the hassle of reinforcing the whole building.