Over the years, my family (wife & kids) have prepared several dozen meals with our cookers on our Model-T excursions with great success however there IS a learning curve that needs to mastered. I will tell you all I know about it (which as you can tell ain't much!!

) but if someone wants to be successful in doing it, it is like anything else in that you need to think through what you are doing first. This starts from studying the roads, planning the meal, determining the length of time needed, and being realistic in expectations.
In the case of the vegetables above, the cooker should be partially filled with water and the vegetables would be placed in a freezer bag made for boiling. Depending on the cooker design, the carburetor/ignition settings, and the terrain, it will likely take about 30 minutes of driving time to bring the water in the cooker to a simmering boil. At that point the food is ready to begin cooking. If someone were going to cook vegetables on a stove for 30 minutes at home, then I would drive another 30 minutes cooking and then pull over to check them for 'doneness'. That places the total cook time at about one hour to one & a half hours max. The same thing with hot dogs and chili. The key is being able to control your heat on the engine's manifold. If you have some downhill grades or long flat leisurely runs, then the fuel mixture and/or the timing needs to be retarded slightly to increase manifold temps.
Another for example was Pennie & the kids put some Bratts & fixin's in one cooker, and some Pork BBQ in another just before we were to drive to the top of Mt. Rushmore in S. Dakota. We placed the cookers on about 10 miles away from the mountain and when we got to the base of the mountain we stopped and checked the water. It was already simmering. Now we made the hard pull up the road into the park and you could smell it was cooking all the way up the climb. Once on top, we checked the water level & temps, and then let them set while we went sightseeing. After an hour or so, the water was still warm but not hot. We topped off the water and headed down the mountain. After about 15 minutes of driving on flat ground, the water was back to a simmer and we found a roadside park to have lunch. The food was thoroughly cooked and tasted great. My point in this was the climb up the mountain was calculated, the time we were at the park without the engine running was calculated (although the water still remained hot for awhile) and the drive down the mountain was all calculated in by the ladies.
Other things with manifold cooking is while items like meatloaf and chicken can be prepared in a cooker dry, those are items best left for someone who has some experience in manifold cooking because the way you learn to do this is from experience. This would be much akin to cooking in the kitchen at home. Those that we consider good Cooks start with learning how to cool simple items and perfect their craft before embarking on 4-course elaborate meals.
As for how to make them, stainless is by far the preferred to fabricate them out of. Being able to remove them from the engine and move to a table is a nice feature to help with food prep and post clean-up. Making the bottom of the cooker conform to the shape of the manifold helps with transferring the heat from the manifold into the cooker. While heat radiating from the manifold will warm the food, it is not the preferred method. Hope this helps some.