FWIW, when I got my first griddle, it wouldn't fit into my oven either. Didn't have a gas grill, at the time. I seasoned mine, simply by using it. Set it over two eyes of my stove and cooked pancakes on it. Kept it well oiled during use with vegetable oil. Don't remember which one it was now, but it was probably corn oil. Kept a light film of oil on it between uses.
It took several cooking sessions for it to eventually turn black all over. Had a problem with the pancakes sticking the first time or two that I used it. Today, I even fry eggs on it without sticking. :)
You could do the same thing over an open fire. Set it up as if you were actually cooking on it. ie bed of hardwood coals, not leaping flames. Use rocks, bricks, or whatever you have to keep it above those coals. Again, as if you were actually cooking on it. If you take that approach, you'll get it seasoned. Don't forget to keep the backside oiled, too. Even though it'll burn off most of it during use, some will remain each time. In time you'll build up a good seasoning there, too. If yours is a two sided griddle, just alternate sides, even if you have to alternate the type foods until you get it well seasoned.
Hope this helps.
Lee
