I'm considering my first sour, too. Already bought some WLP665 and have the malt in hand to make 10-12 gallons. I'm really leaning toward using one of the blue poly 15 gal. barrels used to ship malt extract. I know of concerns about O2 permeation, and have been thinking about preventing it during long-term aging. I'm trying to control costs (poly barrels would be cheap for long-term aging). O2 is a concern of mine. I'm a physicist by trade and _think_ that a body ought to be able to cut the O2 diffusion rate by at least an order of magnitude with a wrap of heavy-duty Reynolds Wrap, taping the seams with aluminum duct tape. Any necessary tear protection could be made with some canvas-backed tape on the outside/bottom of the barrel. Recognize that I'm still in the speculative stages, I have two of these barrels an may look into getting more. Anyone with experience? Sours seem to like multiple years of aging, and this looks like a really affordable route compared to wooden barrels or big glass carboys. But I can see a carboy being good.
I use such aluminum foil to make light shields for my ~5-gallon carboys. The aluminum tape I mentioned ought to stick to anything of interest, and the long passages along the adhesive layer should prevent O2 permeation through the seams. It would be easy to cover more than 90% of the barrel surface. The dead space between Aluminum and the barrel holds a small amount of O2 compared to the head space, and a body _could_ vent the fermentation in place through the fermentation lock via a tube down into the body as a CO2 flush. A second fermentation lock _could_ be plumbed in as a final vent if one wanted a "belt and suspenders" approach with essentially 100% coverage of the barrel.
Thinking and soliciting feedback. If it's a good idea, maybe it'll help someone else, too. If not, I'd like to know why. For example, maybe there are sufficiently cheap sources for 15 gallon bottles that have eluded me.