Roasting (vegetables) on a Rocket

As food-preservation time is coming around again I've been experimenting (again) with using an outside cooking rocket stove for roasting (mostly aubergines and peppers). I've tried:
1. placing a metal surface on top of a simple L stove.
2. placing a metal surface on top of two simple L stoves.
3. placing a metal surface on top of a full height J stove.
4. placing a metal surface on top of a L stove with an expanded top chamber that is sort-of-sealed by the metal surface except for a small venting opening.

I've tried using a 5mm steel surface and I've tried a very thin (don't know what kind of metal it is, old roofing lying around) surface. The thin surface seems to perform slightly better ... but nothing to brag about.
In both cases roasting works best right on top of the flame path and performance drops rapidly when moving away from it.

I feel like I am failing miserably ... at least I think I am. It works but takes a long time.
Any ideas on how to make this work better or should I be reverting to a grill on top of an open fire?

Ronen Hirsch : This is an often recommended trick, make a large dome out of multiple layers of scrunched up Aluminum foil big enough to set on top of your barrel shinny side
to the inside, This will improve your results enough to get you to look for the 'camping Oven' that fits on top of a two burner Coleman white gas/propane camping stove Big Al !

Ronen, i've resigned to use a batch rocket for cooking. Quite crap used as a barbecue, but plancha style, with a heavy cast iron plate works wonders.

Quick view of the latest one.



An older version L tube, showing the cast iron plate i still use.

http://www.youtube.com/watch?v=Paj4hiZgGc0

The batch one is definately simple to build, and and working fantasticaly.