Several years ago I used rebar, cement and a bucket to make anchors for a floating golf green. I bent the rebar like an omega shape in the bucket of cement and when it dried it worked great. Two of those would do the trick except that it could still be carried out by a few people. Attaching them to my cheap bike rack would work.

Of course all anybody actually does is reduce the chances that your bike will be taken. Very difficult to eliminate the chance in my situation. Moving the numbers in my favor is all I can do. Start with either the cement buckets or cheap bike rack then add the other and it's a fair solution.