I don't send proposals. Clients come to me with what they want, we send a few emails back and forth to clarify things, I build it. I charge an hourly rate for however many hours it takes me to build it.
For clients where I've worked with them for many years, these are just a waste of time. I know they'll pay me. They know that I'll deliver. I am careful to not agree any work that I won't be able to do. And they are very careful to not ask me to do work that they can't afford.
If they really can't pay me, then I know they've went bankrupt, so no amount of contracts will make my invoice recoverable. I accept that as the cost of doing freelancing.