Following up on yesterday’s post, I promised a pic to show how much space I have under my seat:

A 20 liter dry bag sitting on top of my rear rack.
That orange blob is a Seal Line 20 liter dry bag. That’s 40% more volume than a Revelate Viscacha for 1/5 the price (not counting the rack, but I already own that). This is why the dry bag solution is so appealing to me.
Note that I ride the largest Pugsley frame (22″) and I still have that much seatpost sticking out. I am a big guy. All my gear is big too. My sleeping bag and pad are extra long; my clothes are XL or XXL (tall, of course) and take up more space. My shoes are size 15 and I need a spare pair for when I am off the bike and trying to pass as a normal human. Where am I supposed to put all this stuff? It’s hard for me to imagine fitting everything I need without resorting to something like this.