When you say "bolt into" do you mean that bolts go through it and then you have nuts, so it's acting as a backing plate? Or is something threading into it, like a screw?
If the former, I would use pre-made fiberglass board, myself. (www.mcmaster.com
, the plain green board is fine) Or I were going to use wood, I would not use a soft wood like pine.
If the latter, I would be tempted to change it to a through-bolted style, since it's door hinges, or to tap a piece of the fiberglass board (for tapping I'd use the red stuff), and then use a machine screw.
Mind you I'm saying this not knowing exactly how your hinges work. Can you describe it more?