Are you planning on having it upright or horizontal? I have 2 of the plastic barrels on stands in the basement used to collect RODI waste water. One is upright, the other is laying on it's side. Both stands they are on are built using mostly 2x4 lumber. For the one laying on its side, I used two 2x6 sections with an arc to match the barrel curve cut into them. At the narrowest point, the 2x6 are 1 & 3/4 inch and this seems to support an almost full barrel no problem. I also added a strip of old carpet padding on the arcs of the 2x6. The barrel is never completely full, since the RODI waste drains into the top hole & would overflow if I didn't turn things off in time. Lose a bit of capacity this way, but it allows me to use the existing holes in the barrel without drilling more. The screw caps for the barrels have a threaded knock-out, so it's easy to plumb a valve to them.
Here's a pic. Doesn't show the stand(s) unfortunately. Time for some more photos I guess. The upright blue barrel doesn't get used much since I haven't drilled holes in it & when I do use it, I have to siphon or pump the water out, which is a pain.
Thought I had some documents on my hard drive regarding wood strength, but unable to locate. I'm sure it's available on line somewhere, that's where I got it. There can be a significant difference in wood strength based on species, i.e. hardwood vs softwood, clear, knotty etc.