Your best bet may be fabric-wrapping your own. Some plain crescent matboards come as large as 60x104, but you would just need a 48x72" or to splice mats together to get the size and shape you need. If you aren't already familiar with Franks Fabrics, check them out. They could make the whole mat for you but shipping may get outrageous. They have tutorials on how to fabric wrap mats so it would be more reasonable to just order fabric from them(or another supplier) and get an appropriately sized mat from a regular distributor, then glue it and wrap it yourself.