Hello,
very good that Jensen sells the fireboxes with the brick imitation as a spare part. This is actually not good at Wilesco. Especially the fire chambers are heavily stressed with frequent use. However, with prudent use of the machines, this should not be a problem. My problem with the brick immitation plate is that I need it for a pair of my 90+ year old Doll & Co. Machines. This then becomes difficult to impossible. I did locate a company in Cologne, Germany that could make the plates for me, but at what price. With my small purchase quantities unaffordable. 
Hello, I do not know your exact needs, but could it be possible to watch the used auctions such as ebay for a used perhaps larger model Wilesco/Jensen or other? The Tin embossing bricks are quite thin and not so hard to work with. Perhaps you could purchase a "parts" machine that is broken or missing major components at low cost and remove the "bricks" and cut down or form them to a new shape to suit your needs?
I have thought of doing this for a custom build....just a thought.