I like diy projects. They are great if you have talents and lots of extra materials laying around.
Have you decided to keep your current design or are you starting the second design?
I have a smoker designed like your first one. (With the fire mounted under the main) It works quite well. I think the radiant heat helps maintain temps.
I added a fire grate to the bottom area to build fire on and use a charcoal basket at times.
I can not tell where the smoke enters the top area.
In my pit it entered on end and was open to grate.
I lined the pit with firebricks to help hold heat, takes a little longer to heat up but stays warm longer.
I don't think a reverse flow would change a lot in your pit. I could be wrong.
It would be easy to set up with 2 rows of firebricks and a lid. I think the aluminum is fine to use. I would use 1/4 inch plate scrap and save the diamondplate for a nice shelf on the outside.
There are a lot of experienced builders here. Ask away and they will all chip in with help.
Keep up the good work.