When a chimney does not have to be aesthetically pleasing then wrapping it with a rubber membrane will be the most inexpensive way to seal it.
Chimney membrane wrap.
Your website says your chimney surrounds are designed to cover a chimney pipe but can they be used to wrap around an existing rectangle chimney.
Description the mfm subseal 40 is a self adhering 40 mil waterproofing membrane that is composed of a polymer film with a layer of specially formulated rubberized asphalt adhesive that is suitable for use on siding and exterior plaster.
On june 21 2017.
There are not many roofing contractors that will offer such a service but anyone that knows how to do a torch down roofing system should be able to do it.
Unfortunately we no longer have access to the product you are describing and only the complete surround kits to conceal chimney pipe.
In salt lake city ut chimney caps are available from the pros at our local fully licensed and insured company.