Wish I had seen this earlier. I have a large warming box on my reverse flow 500 gallon " Big Boy ". I use it to pre-heat and dry up to 12th of a cord of wood (temps are in the 20s at 2am here) before I add it to my firebox. Once it gets nice and hot, I pull it out as I need to add wood. By the time Im running low on wood in the box, the outside temp has come up into the 40s, so I pull the rest out, put it on my grate floor of the trailer, quickly brush the grates in the box, then wait for the goodies to be wrapped. Once wrapped, I throw most of it in the warmer.
The box itself is about 4 1/2' x 3'x 2 1/2' (guessing, its snowing hard right now, Im on the couch reading) . I have a sliding door with a "plunger control knob" between my cook chamber and the warmer, and can get my temp to about 95% of what Im seeing in the cook chamber when the slider is fully open. Close it up, and I can keep it down around 140- 150, depending on my fire, and the outside temp. I put the wrapped briskets and butts in stainless hotel pans, foil the top, and can keep them right at 140 for a long time. The warmer is also a great way to cook a lot of extra food, with the sliding door open. I get full smoke in there, and have a separate 4" chimney that goes halfway down into the warmer, with a sliding rain cap/ damper to fine tune how long the smoke stays in there. Its great for a couple hundred wings, or sausage links (I have some sausage hangers that go in there, haven't got around to trying them yet) Another thing I haven't tried yet, is adding some wood chunks to the floor of the warmer (they will smolder there) to run another wood for flavoring different foods. Smoldering wood isn't the best smoke, but it might work out OK.
The way I see it, if you're spending the time and $$$ to build a reverse flow smoker, why not add the warming box? The cost is minimal, when compared to all the rest of the materials you'll be buying. I can provide some good pics when it stops snowing, of how my slider works, and the general design. Its really simple, and it just plain works. Im building a 320 gallon smoker trailer this summer (back problems solved hopefully), and will probably use 2 100 gallon tanks for my warmer and firebox on that rig. The firebox being horizontal, and the warmer being cut to fit so it stands upright over the cylinder, if that makes sense.
And, to agree 100% with Smokejumper, DO NOT put food in to "pre-warm", unless you can get it up to temp like mine does. I wont run mine below 220 degrees with raw food in it. I usually run 250-275 with nice clean smoke during my entire cook. With my sliding door open, the warmer will run 5 degrees cooler on average, which is still good for what Im doing. The last thing you want is for you, or your family to get sick. ill try to post some pics on here this afternoon if the snow ends.