Thats why you should spot it, a full weld would probs warp the metal, Im not saying just a couple at the corners, just one every 2 or 3 inches will be fine, as for it not being strong enough, your whole car is held together with spot welds.
As long as its strong enough the plate wont flex same with the boot, and you need to get the plate a perfect fit then you wont need to use much filler, just a thin line.