I believe one of the push goals for the rocket oven kickstarter was a J-tube
rocket stove grill top? But I second getting Erica and Ernie's Builders Guide. The J-tube section of a
RMH is a
rocket stove; make sure it's insulated but just the bricks for that section and a cooking surface on top of the riser that's stable and allows exhaust gasses to flow by will do it. Matt Walker has designed a brick rocket stove/oven/heater as well which you could start with just the stove/oven portion, which would incorporate some mass into the design for heating.
But for just an insulated burn tunnel and riser for cooking, I would set up insulated fire brick, and then surround that with additional insulation and reinforce the riser with a wider, more stable frame.