I'm going to resurrect this dormant thread with a non-UDS post (not that I don't love UDSs, but old school barrel grills can always use some love).
I've been making a number of straight up horizontal barrel grills over the last 20 years and feel like I'm just starting to get them just where I'd like them to be.
The main concept I'm currently using is just a barrel with 1/4 of the side cut open to make the door, and then using a Vogelzang door at one end to allow access to the fire without losing all the heat:
I cut the door into the side of the barrel, bend and attach some 1" flat steel to make a better seal on the sides of the main door, throw the Vogelzang door on the front of it, and make a charcoal grate out of expanded metal and angle iron to hold the fire:
I also add a pipe at the exhaust end to keep the exhaust at grill level, thereby keeping the heat and smoke inside of it while the food is cooking:
This is not an ideal rig for smoking, but you can certainly use it for smoking. You can smoke on the left half of it for extended cooks (as long as you're willing to stand by it and feed the fire every 30 minutes or so - beer in hand of course), but a UDS, WSM, or an offset will be a better bet for long smokes. That said, it is really versatile for everyday use.
It works incredibly well for roasting and grilling. You can build a large fire that covers 1/4 of it, and then have a wide variety of temperature zones to choose from. I usually get 1/4 of it screaming hot to sear food when I first put it on, and then move the food off the direct fire to roast at 300-400 degrees as needed.
I've been tweaking this one for a couple years now, adding things like a griddle and building a new stand. My latest addition is a 1/2" steel bar grill that I welded together. It does a great job of searing anything it touches!