Welcome to the group Kolbster310!
I can't help with sealing the firebox. Mine fits pretty tight, so I haven't messed with it yet. But I may check into the sealant Lemans mentions.
Lemans has you covered on basic mods pretty well.
I did most of my mods before ever firing up my CG. Sealant around the cooking lid, two therm's on side, and etc. Easier to get stuff to bond with new clean metal, than it is after grease gets in there. Ya know?
If I had to do it over, I would skip those two side therm's, and just check with a digital unit with cables. The therm's seem to get out of adjustment very easy after calibrating. . I've quit depending on them at all, but have to leave them in because I drilled holes for them. And it still look cool enough to impress family and neighbors. LOL
Extending the stack down to a couple or three inches over the grates is a big help too.
And buying that thermal welding blanket has done wonders for me!