"I'm sorry but we are unable to produce concept designs for you as we are a busy company. We understand why you would ask for a concept design but due to safety issues we only take on work that we know that we are going to be paid for, this is also the reason that we require a 50% payment upfront so we are reassured that we will be fully paid for the work."

I tried, I got a bit confused half way through and perhaps it doesn't make sense, but something like this I think would do