You need 100% of the payment up front. 50% is only going to cover your supplies and not your labor or any profit. If you are making to size, you may not be able to sell them. that is 100% up front.
If you give someone a quote, and you don't hear back from them within a couple of days. send them a second convo, and after that, they are done. don't contact them, they are not serious. And NEVER start an order without payment. Don't start one on a wish and a prayer!