Pre heater and hood length?

[StayinLowTech]

I am building a pre heater and hood for my flue pan. I like to skim the crud at the very back wall of the flue pan. Would I lose a lot of efficiency if I stopped the hood about 6" from the rear wall of the pan and put a piece of sheet stainless down from the back wall of the hood to just above the level of the boiling sap, allowing room to skim? I guess some steam and pressure would be lost as the hood would not be as tight as when resting on the top of the pan all the way around. Any thoughts or first hand experience? Thanks for helping me make up my mind.

[wiam]

If I open the door to my hood I can't put my hand close enough to skim.

[maple flats]

I used to skim too on my older evaporators, but with a hood I don't get the crud anymore. I think it may be related to the hotter environment under the hood.