Welcome aboard!! I would use 2'' minimum thickness for the FB insulation. The BP gap is the same as the throat or up to 25% smaller, I use the same as the throat and haven't had any issues. I build the BP level front to back and end to end, the majority of the grease will cook off.
No disrespect to BigT, but I built my firebox with 0.25 inside, a 1 inch ceramic insulation (with R factor rating than 2 inch rock wool) and skinned the outside with 12 g. Couldn’t be happier.
At the end of an 8 hour cook, I measured the inside wall of my smoker (with a laser probe) at almost 750 degrees and the outside skin was only 225. It’s gonna burn you but not cause an instant trip to the ER.
Ditto on the flat baffle plate. Some try to angle it or run a trough thru the middle but it’s just not needed and could be argued to actually interfere with airflow. You will be surprised with how little grease you have at the end of a cook as most of it vaporizes.
Learning generally boils down to "Repetition or the avoidance of pain", some people learn by doing, some by watching and some just have to pee on the electric fence.
Whether you think that you can, or that you can't, you are usually right. Henry Ford
Ditto on the level baffle plate. One of the benefits of a reverse flow is having the drippings re-flavor the meat and it works best if they don't drain to one end and pool leaving the other end to burn.
I insulated all 6 sides of mine...including a double walled and insulated door. I did mine more from a safety point rather than heat loss/fuel efficiency. Figured a person (or my younger son) could just as easily get a foot or hand under the firebox and get burned so it was cheap insurance for me.
Just seeing how to download pics. Been using this for 6 years. Me and a buddy through this together and are ready to upgrade. We have a 250 gallon tank and a 500 gallon tank. He's going to be building the 250 and I'm taking the 500. It's nice to have the info of this site to help out. Thanks