In case it helps anyone: I had a 12,000 piece puzzle mounted and framed by a framing store. I forgot the details, but they used something called "Gator Board"

In case it helps anyone: I had a 12,000 piece puzzle mounted and framed by a framing store. I forgot the details, but they used something called "Gator Board" (they could get it 10-foot sheets, along with a 10-foot long piece of plexiglass) and they used a machine the heat up the adhesive in sections. They used two separate sections of Z-channel which is apparently what museums use to hang frames. And they advised that I use washers to line up the Z-channel since most walls are not straight. My original plan was to glue it to a single 10x6 piece of plywood that I could get custom ordered for $300, but even though the mounting and framing cost me $2300, it was well worth it (I got a really nice gilded frame).