The funny thing about the "Honey Cow", other than it looks very similar to mine, is that it is all staged pictures and no bees in it. :roll:
nietssemaj - I know exactly what you want to do with the modified frames. I started my first TBHs the same way, only with deeps. I'm pretty sure a medium frame will fit in the half barrel but can verify it tonight for you. As far as screen in the bottom, I'm not much for adding excess ventilation, but then again my climate is mush different than yours. I spend a lot of time observing feral colonies, and at least in my area, they do everything they can to seal up everything but the entrance.
However, if you want to add ventilation in the bottom, I would suggest cutting a series of large holes with a hole saw and them gluing or screwing #8 hardware cloth over them. This way the structural integrity of the barrel will not be too compromised.