Two things to consider when making a top plate, which I have done is that a; it cant be totaly flat because the top of the rear gearbox and brake seat is higher than the front gearbox.
and b; If you want a top mounted servo (one that you can take out without removing the top plate) and a servo thats not a stressed member, then the top plate must contain parts that runs vertically along the sides of the servo because there is not enough width to make a top plate thats wide enough to drop the servo into and be strong enough.

CEN has designed the Magnum quite well in that respect, if you download the magnum manual you will see the new top plate design, or check pics below, but it also has a new brake seat with mounts for the top plate about 2 thirds the way up so. For those with standard chassis it will probably fit (with the brake seat as well) but for those with JT the easiest way is to put an aluminium rod through between the shock towers.
It also includes on of the mounts for the lay down servo!