I agree with getting a beater bike at Goodwill or a garage sale for under $100. And don't get the bike that looks nice, get a crappy looking one, or make it crappy looking; and don't clean it.

As far as locking goes, I understand the idea behind a small U Bolt being harder to pry but with those you can only lock the rear wheel and the stay to a post. In reality the better way to do it is to get a U lock that is long enough so you can take the front wheel off move it to the rear alongside the rear wheel and lock the front, rear wheel and stay to a post with very little space on the lock left over. If the U lock as very little space left after the bike is locked in this manner prying is still difficult.

Then get a second lock, either a large diameter cable or better yet a heavy chain, and lock the entire bike wheels and frame using a Discus lock because those leaver no room for a bolt cutter to get at the shackle.

Use cheap components so they won't hopefully steal those.