I have no horse in this race, perhaps I could lend some insight.... Ymmv
In the sketch - am I to understand the cover is made air tight using a water bath? As indicated by the blue squiggles ?
While this water bath in a open air example would be a good air tight seal, there are two places that this is not true..... When the positive air pressure in an enclosed vessel exceeds the water column height of the water bath. And when a negative pressure is applied that exceeds the water column height of the water bath. Both would cause burning in the fuel chamber.
Now if there is no water in the water bath you have an air leak.... And again burning in the fuel chamber
If you have no water and use a blower, you run the chance of creating a Venturi effect, sucking air into the fuel chamber as air passes from the blower to the cooking chamber. This Venturi effect grows stronger as fan speed increases. This effect is also caused by the draft that is created as heat builds in an enclosed box that has a exhaust stack.
I've noticed that woodpelletsmoker builds smokers that are small with very short exhaust pipes or in one model no exhaust, just a few small air gaps.
I only skimmed the other posts , but if I recall Bobbyp's smoker was an offset with some long exhausts on it ( correct me if I'm off ). But something like 6 feet (183 cm) of pipe 6 inches (15.24 cm) that would create a lot of draft at temp.
I do believe that the cover fitted with a silicone gasket - well really a flat gasket fitted in the bottom of the water bath area would eliminate most of the air leaks that could be created. Additionally two clasps to fasten the cover firmly down to the proposed gasket would reduse the positive pressure issues and insure an air tight seal.
If woodpelletsmoker could get some pellets or someone send him some it would save a lot of discussion on it as I do believe everyone involved wants to see a product that works .
But really I think the issue is air , draft, and larger cookers than Woodpelletsmoker expected. I do not think the fuel is the problem, it just shows the effect, but not the cause.
If anyone has one of these mounted and having problems make a gasket ( I do not think stove gasket will work here ) of high heat silicone and weigh the cover down and try it - I bet the results will be better Then find a couple of latches to get the cover in place once you have confirmed the air issues are gone
Again this is only constructive comments to try bring improvement in what I think could be a decent product. As well as assist anyone that may have one of these and having issues.