Rising makes a warm white 4-ply matboard in 60 x 108, if you can find it. I've had success mounting some small pieces to MDF. I had a customer that liked to mount photos to MDF and arrange them in 3D collages. I used Fusion 4000. I'm pretty sure you can get MDF in 60 x 96. You'd have to cut it on a table saw. That would be one HEAVY poster, though!