How to make a wider cooking surface for a rocket grill?

I built something very similar to the rocket grill shown here:

  https://www.youtube.com/embed/ZBmJuuK20YE  

And it worked perfectly!

But all the heat is concentrated right over the middle. The guy in the video is using a big metal plate, so that the grill is more like a big pan to distribute the heat. I'd like to cook over the fire/coals, but I'm trying to figure out how to have a hot spot bigger than one hamburger.

My best idea so far is to put the bowl of a charcoal grill over the hole and attach it somehow. I figure the heat will have to diffuse into the bowl and then the whole bowl will be pretty hot.

Does anyone have experience with this or recommendations?

Cast iron would be my go to for a cooking surface. A flat skillet that fits the opening would work well. Also have you tracked the temperature and time on the cooking surface? Something like the following: started fire at this time, cooking surface was blank temperature in the middle and blank temperature on the edge. Ten minutes after start temperature was blank in the middle and blank on the edge.

Hi Joshua,  I found this post by Matt Walker that may be of help:  Rocket-mass-oven

Different than what your looking for I know but may spur another idea.