60" photo.... can you even find oversized acid free mat board?
I believe so, though I've not made note of the oversized Mats I've bought from what was a Cheap Joe's here in Charlotte, in the past.
40 inches by 60 inches are common, and other than buying them wholesale, my only other recommendation, besides type 100% cotton rag, alpha mat musume board 4-8 ply, (the heavier stuff is much more expensive, thus the need for a wholesaler or tax number), is, if you need to add extra width to the mat, use the same matching mat board (from another sheet) and cut overlapping full hight, long panels, for both sides, and use whole wheat starch, NOT Flour, to first butt joint the new up front adjoining panels and when flush even to the rest of the front surface, cut reinforcement panels to join and physically support the back side panels in place.
Cut more full height, or long, support panels and add these to the back of the front surface.
If you've enough mat left, three cut pieces in the shape of an "X" between the added on each support panels, cut soundly and square to the back geometry, including edge gluing to better support and handling strength.
There's a lot to doing it, expense and labor, this way and you might seek other advice, but because I try to offer first, best practices to my mats, this is what I recomend