Some of the woodworking magazines have published plans over the years. I built mine (4x4x8) out of plans from American Woodworker #94 June 2002. FWW I know has also published plans. As other have mentioned, there are threads here dealing with kilns also. I built mine with a bunch of fancy control circuitry, heaters for lightbulbs, an attic fan, and a DH. Now, I just use the DH and the fan, don't bother using all the fancy circuitry.
12x8x4 will be a nice size, I guess kilns are like shops - never big enough! I air dry my wood two years, then put it in the kiln with the DH running full tilt, and a hose running out through the side of the kiln for the DH to drain. I collect the water in a bucket and keep track of how much is removed each day. The heat from the DH is enough to increase the temperature in the kiln to over 40 degrees C. After three or four weeks it usually gets down to less than 1/2 pint of water removed a day, and then I turn on the fan. That seems to raise the internal temp of the kiln over 50 C and draw more moisture out, but only for a few days then it is done. It usually gets the wood down to about 8 percent or so. I used to condition the wood afterwards by blowing steam in with a carpet cleaner, but have found no value in doing that so I stopped. I think the wood being air dried for two years minimizes the amount of stresses in the wood.
Make sure you insulate it, I just used 1" and 1 1/2" foam board insulation from the borg. I bought a cheap temp and humidity gauge that has remote sensors on it, I put the sensors in the kiln and leave the gauge part sitting on top of the kiln. Similarly, I use a pin type moisture meter, I just drive nails the right distance apart into a couple slabs and run wires from them outside the kiln to clip onto the moisture meter. So there is no need to open the kiln to drain the DH, check the humidity or temp, or to check the moisture content of the wood. Of course, some times I just open it up to 'take a look'!
Yours will have almost 4 x the volume of mine. I wonder if a single DH will be enough? I guess you can try and see what happens, always add one later if it turns out you need one.