2x4 hobby evaporator

[Diesel Pro]

I have a 2x4 WesFab hobby evaporator. The design I would best describe as having a deep throat. It is already 13+" deep at the mid point and is 6" deep at the very rear. I've been reading about how it is best if the volume is reduced behind the firebox. I've seen guys adding sand etc. As is it is working OK, but I wonder is it worth trying to fill in the throat area?

[5 Oaks]

I was using a 2x4 supreme pan, drop flues. I added brick to the back of the box, I have about 3 inchs of space between brick and bottom of pan, it made a HUGE difference from when i had about 6 or 7 inchs.

[WESTMAPLES]

i have a 2x6 drop flue, i have from the top of my firebox ramp all the way back to the stack double bricked so that the drop flue nearest to each outside has no more than a 1 inch gap on the side and under the bottom of the flues there is not more than a half inch when seated. it keeps all the fire/ heat pulling right thru the flues and not thru wide open spaces. i learned this from others here on mt, and for the material you use to fill it, ive seen sand, peastone, traprock, and even plain dirt used for a cheap solution, i just used more of the half and full firebricks because i had them, others use fire blanket and fire motar.