My Teeny Tiney Pig Roaster I'm building...

Controlled air flow.  You need it for the fire and you need it for the cook chamber too. You gotta have vents and/or a flue.

BDL

Vents and a chimney/flue both, I'd say, for full control.

I see a hole in the left side, top. You could attach the flue there, with either a hinged lid or butterfly valve, and put a similar hole, with butterfly valve, in the lower half, on the right.

Add a side box, so you can use it as a smoker too.

Maybe a series of ledges on the top, to hold coals so it will work like a la caja china.

Dude you could probably market this as like an all in one thing.

That looks like a former liquid barrel, if so the bung hole in what used to be the top should perfectly accommodate a 2" threaded cast iron pipe. Things to consider, the ultimate goal of the flue is to let smoke/CO out, while retaining heat. If you locate the hole for the flue lower, that is close to the level of the food grate, and then place the vent holes on the other side of the grill below or at the level of the fuel grate, as long as your flue stack is tall enough the air will draw across. This will allow smoke to escape and hold heat in, which rises to the top. See pic of the grill I built last year. Unfortunately it looks like you already cut your barrel so the bung hole is higher up. You can control the temp of the grill and the rate of charcoal burn if you create some adjustable cover to the lower vents. There is no need for a vent cover over your flue unless you want to kill the fire so you can save unused charcoal/wood for later use. Seriously, their is no more wholesome redneck fun than smoking some good ol Bar-B-Q on a homemade grill. A country boy can survive, and eat good. (I know that's not proper grammar)

That's a neat rig. I have one very similar that I use for doing oak fired grilling. What I found to be the biggest issue was air flow, and it was necessary (at least for grilling) to create a grate at the bottom so the wood would could get air flowing underneath, in addition having ventilation holes drilled on the bottom sides. We drilled holes in the top and could close them off to act as a flue.

Just as an alternative, my buddy and I have done a couple pig roasts, and elected to use cinder blocks for a couple reasons. 1. You can easily tear down and store the pit when it's any one of the 360+ days you are likely not roasting a pig. 2. The block comes up to heat and like a clay oven, it creates an incredibly stable heat environment. The first roast we did was on a very cold day (for Florida) the wind chill was the worst part coming across a lake, but we fired up the coals and once the temp got up to around 255 we only had to add a little coal about once an hour. Here are some shots from that day