Hopefully I can answer all the questions, although I am away form my pit right now so will have to double check on a few of these:
1. Any other mods since?
Nothing major, but I have been playing with the placement of the vent. Once issue with this design is getting the heat to evenly distribute from top to bottom. It distributes nicely across, but I have abiout a 30 degree difference between the grate and the lid. Not terrible, but I would still like to improve that. IN an attempt to do that, I dropped the vent to below the grate to allow the heat to build up and reverse back on itself a bit. Optimally, I would like to get a steel plate and turn this into a reverse flow build which was the original design.
2. I need a good lid, other than the piece of old ply I have been using. I am not sure about the one you made, but I will look into it. I have seen folks using cement board.
The lid I have is great at holding in the temp, but you need to have a metal bender. I had access to these at a local shop. The thick aluminum keeps any contaminants from the ply out of the food. The rockwool/mineral wool is a great insulator that is used in commercial smokers and is completely chemically inert. If I would make mods to the cover, it would be to make it lighter. Use 1x3's instead of 2x4's and thinner ply. I used 3/4" and overall, the cover is a beast, but I am barely losing any heat. The combination of the rockwool and poorly conducting wood works well.
3. Placement of the exhaust vent?
I have been playing with this a bit. WHen I did the pig in the pics, it was placed right at the level of the grate and the pig came out perfect. I did another one when the vent was just below the grate, and that also came out nicely.
4. How low did you go?
Related to temp?....I kept it around the 230 degree mark.
5. How tall for the stack? did it make a difference?
THe stack I have there was the standard 48" (I think) vent pipe. THe diameter is about 5" and is plenty for the smoker. I have a baffle in there that I can close almost completely to control it. I would say it doesn't need to be this high to get the flow you need. All of this is available at Home Depot.
6. What about fire brick in the pit? I do not know how expensive it is or how hard to find. But I figure it can help hold the temp.
I just used regular brick. THe concrete is what you need to be concerned with cracking. Even regular brick is already fired at temps much higher than what you are going to encounter in the fire pit here. It does help hol the temp a bit, but mostly I wanted to insulate the concrete block form the temps in the fire box.
7. how big is your firebox?
Not big enough. THis is the one mod that I need to work on more. The current box only has internal dimernsions of 24" wide x32" long x 16" high. It's a bit hard to calculate hte size needs for a firebox with a concrete block cooker using traditional methods. Since most steel cookers are calculating jsut the internal size fo the cooker, that's all that you have to account for. However, with this, you have to consider the fact that you are also heating the thick walls of the cooker. The length is more than enough, but I would mak the box a bit wider. It would require a mod to the opening as well to get the heat right in there. If I get around to making the reverse flow concept, I will have to put some pics up. To save on money and time, I have used a 60K btu burner from my crawfish cooker that I will jam in there to preheat the cooker. I will usually turn that up good and high and llow it to run for about two hours before I even start with the wood and charcoal. The $16 refill for the propane was much cheaper than the multiple bacgs of coals that I used to previously warm the body of this cooker up.
Hope this all helps and let me know if you have any other questions.