Personally, I’d just work to improve its wind and waterproofing. Add as little weight as possible. Put more bracing supports in. As others have said, there’s too many obstacles to making it a ‘garage’. Best to consider what functions you want it to perform and what the problems are currently and work towards fixing some of them, ie, “I’d like to be able to keep stuff dry” or “it would be good to be able to work on stuff in winter”. If “I want to securely store stuff out of sight of others” is your goal then that might be a stretch.