I agree, 1/2 upon ordering it, have a contract stating that and the other 1/2 upon completeion. Write out the $ amounts paid and the $ amount needed upon completion. Also, in that contract state that IF for any reason they change their mind and cancel the order - at least 1/2 of that 1rst 1/2 is NON refundable. And, if AFTER completion they change their mind - then deposit of the 1rst 1/2 is NON refundable ( you have already done ALL of that work!)
Most people will not back out after the large deposit has been made.