how do i make an overlay like a pumpkin

so i have made an effect that makes you 6 blocks tall with the pehkui mod but i want an overlay effect(sorta like the one you get for wearing a pumpkin) but i want it to be like a an image of a red border but the closer it gets to the middle of the screen the more translucent it gets how do i make an image like this that covers the whole screen instead of a tiny little bit
ik how to make all the procedures and stuff like that i just need to know how to make a overlay image cover the whole screen