Excellent job on that fire door Ron! Looks like you did a fantastic job. I have a bit of a different arrangement on my snp which is aolder version of what you and John have. I works pretty well but dosen't choke off entirely. Here is a pic of my set up.
Also I know what you mean about a sheath for the bottom of the firebox. I ordered some heat shields for that purpose from Brinkmann awhile back. They mistakenly sent some extra so if you want one pm me your address and I will be happy to ship one out to you. Here is what it looks like:
Lays nicely in the bottom of the firebox for a little added protection. Now that I am using the snp as more of a stick burner I have given some thought to making a burning barrel. When I initially start a fire in the firebox of the snp the top gets VERY hot from all the flame. To the point of I think it may be warping it some.