Bike storage shed, preferably lockable (use for riding tack) (Boswall EH5)

My daughter is a keen rider, and we're looking for a cheap solution for securely storing her tack. After looking at what other riders have around the stables, something like a bike storage shed seems like a good option. We'd like to be able to lock it as well to make it secure. Plastic type may be easiest to find but open to metal type too. Good day time and evening availability for collection. Hope you can help. Thanks.