The gap photo
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About anything you wanna know about smoking/grilling/curing/brining/cutting or slicing
and the list goes on has probably been posted. Remember to post a QVIEW of your smokes.
We are all smoke junkies here and we have to get our fix. If you have questions
Post it and you will probably get 10 replies with 11 different answers. That is
because their are so many different ways to make great Q...
I don't know if it's just me or what, but I prefer cooking with an offset stick burner. I use just enough charcoal to get the wood started (about 1/3 of a starter can) then wood only. It seems to impart just the right amount of smoke flavor, considering the type of wood you use; some wood being mild, some strong.
I have a smoker similar to your Horizon and did several mods: http://www.smokingmeatforums.com/t/166840/rivergrille-farmers-grill-and-smoker
They definitely allowed me to reach higher temps with less fuel use. I bought the gasket material on e-bay, it was 1/4 X 1" flat wood stove door gasket. I really like the cam locks. I wouldn't worry too much about the door gap on the fire box as far as it causing a smoker problem (the fire has got to have air) as long as no coals fall out and catch something else on fire. Mostly, you are going to control your heat in these thin steel smokers with the size of your fire more than with the vents anyway. I almost always have at least the main fire box vent wide open all the time.
The primary thing you will have to master is thin blue smoke. Your fire should always be burning, not smoldering. When burning wood for fuel, it doesn't take long for creosote to form if you close down your fire too much, and you end up with brown residue all over your meat and inside your smoker. I've tried it and it don't taste good.
Good luck with it,