This was the original plan.

I am using the vertical chamber from this old offset I've had for 15 years.

I'm using this 100 pound propane tank for the horizontal chamber.

I ran the numbers through the PitCalc and came up with a 16.5 x 16 x 12 firebox

Then I found out that I should only use the volume of the horizontal chamber and not the combined volume of both chambers. ( thanks Rod Crafter ) Now my firebox is way oversized.
I wasn't originally planning on insulating the firebox but I did some more fingering and found that if I insulate, the inside dims of the FB will be 15.5 x 14 x 15.5. That lands right smack in the sweet spot at 112%. so I guess it will work out OK.