dwayne
I am sorry that I have not seen your post soomer, I have been out of action for awhile, surgery, I am just starting to be able to do things again.
I think that the adobe should be in some what of a cone shape. I use charcoal so no experience with coal. I made the adobe about 1"+ thick, the bottom a little thicker, so that it keeps the air flow free moving. As with all adobe it is going to crack as it dries.
I just keep filling the cracks and rubbing them out with damp slurry until it had dried.
Yes the pipe that I used is 2", you need to go into a tee so that you can have a ash dump.
The fan-blower need not be powerful, just a adjustable for volume of air flow.
The holes in my grate are pointed to move air to all parts of the fire pot.
I hope that this helps.
Have fun with your new addiction ( blacksmithing ) and that is what it will become if you
enjoy it.
LeeRoy