MDF is Medium Density Fiber Board. It's a wood sheet you can get from any lumber supply. Looks like a big sheet of plywood, only a lot heavier and no wood grain. It's made from wood pulp and glue. They use it to make speaker boxes. You can buy a 4x4 sheet of it from Home Depot for probably $8 or so. Don't quote me on that price. I think a 4x4 sheet should be enough for all 4 doors. Trace the front OEM speaker bracket on it so you know the shape to cut it. Hopefully you have or can borrow a skill saw:
Use that to cut the outside shape. Then draw the new speaker holes on the MDF piece (make sure there is clearance when installed) and cut those holes out. Slap some black paint on it and you have your very own custom speaker bracket that will hold both the mid and the high. There used to be a template someone drew so you can just print it out and trace it to make the bracket. I used to have it but it's gone after my external hard drive quit.
You can also get a white plastic 1/2" thick cutting board and do the same thing.