Check out:
http://www.barbecuebible.com/
Look for the BBQ BOARD link in the left column. It's a forum dedicated to BBQ.
I do belive there is a section on DIY smokers.
Your objective is to have a fire box that holds enough charcoal/wood to enable you to not have to tend to the smoker all the time. IT has to have controlled air flow to it so that you can get the temperature you need, without burning up the fuel too fast.
The ducting between the fire box has to be high enough on the firebox to pickup the smoke and heat. And enter the smoke chamber low enough to be dispersed evenly.
The grill that you have is big enough that you'd probably be better of with two ducts running from the fire box. Run the duct toward the center of your smoke chamber...and it's also helpful to have a baffle... The baffles job is to catch the smoke and heat as it pours out of the ducts under your grill...and distribute it evenly. They can be simply a plate of steel with holes drilled strategically.
It is SO IMPORTANT...that you try to design your smoker so that the temperature and smoke is even across the entire cooking grate. Your thermometers should be at the level of the grate. You want to know what temperature the meat is actually feeling...not the roof of the smoker. If you get uneven heating...it will cause you to overcook some areas and under cook others. You don't want to have to open the smoke chamber during smoking...so even cooking is KEY!
You can imagine when you've got part of your smoker going at 190 degrees and another section getting 230 that over a period of 18-20 hrs (Beef Briskets) you'll have some extremely disappointing results.
I smoke at 211F most of the time. I can go 4 hrs without adding charcoal. My heat stays pretty even without constant adjustment of dampners, etc...
Ribs take 4-5 hrs, Pulled pork 10-12 hrs, Briskets 18-20 hrs, Prime Rib 2:30-4:30 hrs...
You might have to experiment with the geometry of your smoke stacks, ducts, baffles, and butterfly vent... WHen you get it right...you're going to love the food you produce!! And so will everyone else.
Good luck...You've got a great looking start on the project.
Your trailer reminds me of the Diamond Plate products line. Look through their photo gallery for some ideas.
http://www.diamondplateproducts.com/50_Series.php
They have extras like bins for wood/charcoal, side burners, gas burner for grilling, gas burner for starting the charcoal, charcoal racks for grilling...you name it.
Making your own will give you super flexibility...