Just go get a new top for the cage from a pet store.
Wait till he gets real stressed and get sick with an RI.
Your going to spend alot more than 40 dollars to get him back to health.
When in all actuality, you could have just spent the money before hand to do it right to prevent the issue in the first place.
You did take this animal in as a rescue, and a lot of people commend you for that.
But he's YOUR responsibility now, and you NEED to do what is best for him from now on. It's people that cut corners whose animals end up needing rescued most of the time.