The mechanics are basically the same, obviously on a smaller scale. Depending on how much meat, and the dimensions of the meat and other materials will determine the depth of your hole. For a turkey and pork butt, we go between 18 and 24 inches deep, and as wide and long as you need to so you can contain the meat, without having it touch the sides, and so air can circulate. It's not as important for everything to be below the surface of the ground. In fact, a lot of places in Hawaii do an above ground imu these days - I think they call it Samoan style.
Dry river rock is still important. Don't get anything that has been in water recently. You can usually go to a building or landscape supply store to get stones. You don't need head sized stones for small amount of meat, but you can't use pebbles. Shoot for a mix between softball, and volleyball sized. You'll need enough to line the bottom of the pit in a solid layer (some stacking required probably). Since there aren't any big carcasses, you don't have to worry about stuffing rocks in meat.
To heat the rocks, you can do it in the pit, but we like to keep the pit relatively clean of ash, and burning chunks of wood. You don't want that on the food, and you don't want liquid dripping on burning wood and smoldering during your cook. A shallow pit on the side, or any other fire containment area will do fine. Something that can still contain heat like a cut down barrel works good.
Start with a hardwood fire for hot intense heat. Once you get a campfire sized blaze going, spread the coals, add the rocks in a single layer, then cover with more hardwood. The heat the rocks contain is all the energy you'll have for your cook, so make sure they get good and red hot. It should be easy to see them glowing when they are at temp.
As the rocks come up to temp, you can prepare your meat. Basic seasoning is fine - salt, pepper, etc. Avoid herbs and aromatics - the length of the cook sometimes make them taste funny. You really want a 'natural' taste to come of all this effort. We do wrap butts and turkeys in chicken wire. It makes them easy to handle, and especially with the butt, holds it together.
Also, it's time to prepare your burlap, and natural coverings for the rocks and pit. Burlap needs to soak a while to absorb liquid. You'll also need access to large banana plants, or whole cornstalks (banana being preferred). Cut the banana/corn stalks to foot long lengths, and smash them so the fiberous cores are exposed. These will soak in water as well. Keep the banana leaves/corn husks on the side and moist (no need to soak).
When you are ready to assemble, move the rocks to the pit. It's ok if there is ash on the rocks. In fact, if you can move a shovel full of burnt ash/warm earth into the bottom of the pit, it will help insulate it more. On top of the rocks go the smashed/soaked banana stalks. This creates some steam, and keeps the food off the rocks. On top of this goes the chicken wire wrapped meat - making sure you are over most of the heat. Covering the meat COMPLETELY will be the banana leaves. This is the initial tenting to trap steam close to the meat. If you have enough leaves, layer them so they cover the entire top of the pit, keeping the edges in the pit. It's fine if at this point your pile of stuff is slightly above ground.
Over the entire pit goes the well soaked burlap. It should cover the edges of the pit by about a foot. Then over all of this we put a sheet of heavy duty plastic, to make sure steam is trapped, and dirt is kept out.
Starting from the edges, start adding the dirt you removed to make the pit. This ring will seal your imu. For these smaller cuts of meat, you don't need to cover the entire production with dirt. We usually leave the top uncovered, so we can monitor the cook. You'll see the plastic balloon up as steam and heat build, and we use this as our gauge to make sure things are still cooking. If you have a probe with a long enough lead, I guess you could stick on in there. When the plastic sags down (like a vacuum is forming), your meat has to come out since your temp is below any point of cooking.
You should get an easy 6 hours or more this way. Adding more dirt over the entire thing will extend it. We've gone 8 hours before with temps over 200 the whole time. It's hard to go longer since you just don't have as much energy in the pit, and the top insulation just isn't there. Our meat is always cooked though, so it's not a problem.
If you think 6 hours isn't very long, don't worry. All the moisture that was trapped in the imu will give you a piece of pork that will explode if dropped on the floor (we've done it). Your turkey will look pretty bland since there is nothing to color it, but it will also fall apart when you unwrap the chicken wire. The flavor you get will also be very different. Some of it from the banana, some from the smoke the rocks picked up, some from the earth you use, and a lot from the concentration of flavors in the meat.
I hope you have a chance to try this. It's definitely something you can have fun with. Maybe some Bay Area member will donate their yard for a day so we can all come over and dig it up! What do you say Chris?
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