I have a duo also. here are some things to consider you want but some things to consider.
You need the side fire box.
After that buy some Hi temp insulating tape. (a)Tape the fire box door, (b)the top shell sides and front, (c)and the back.
For the back there is a large opening from the factory.(d) I had to use two pieces of tape on the bottom shell stacked on top of each other.. and one on the top.
(e) On the opening to the fire box for now I have a piece of aluminum foil to act as a heat shield against flare ups.
There are also random holes to patch. From where the handle used to be and from taking the warming rack off. I used tape on some and JB weld on others
I would suggest reading up on the minion method as well.
I made a small charcoal box and fill it high with un lit lump. and about 2/3 of a
charcoal chimney and light that. I pour it in when it heats down to about half a chimney then add to the fire box.
ever since I started doing that i can get it to sit on 225 for about 6 hours with minor mods the the intake vent here and there. The top vent is kept wide open.
(a)
(b)
)
(c)
(d)
(e)
(e)