It depends on the weight & rigidity you need. Ply wood or MDF work well, spray paint the edges matt black, the lighter option is Kappa board, this is a foam board very light. You can also use aluminium sheet.
Either way you use 3M spray mount adhesive to bond the print to the board, making sure you have the permanent type. With wood most sores will cut to size, with Kappa board often people trim board& print after mounting.