I think I'm a bit grumpy because this was supposed to be a "this weekend" project and I think it just turned into "another trip down the road" project.
I could knock it out with concrete blocks or big huge pieces of wood, but then you get into weight and aesthetics.
With the PVC idea, it's just a question of whether it's immovable in relation to the legs or the ground because it has to be one of the other. I looked at those mailbox stakes you can put a 4x4 in, but those are hugely long - it would be a booger pulling them up. As far as making them immovable in relation to the ground, I could put flanges on the bottom and stake the flanges into the ground

That's probably the quickest/easiest route.
More elegant in the vein of making rigid leg extensions might be going with a smaller diameter pipe that the foot doesn't fit in to. You make a notch for the foot to ride down in to - say 4-6" - then drill two holes completely through legs and PVC and attach them with a bolts and wingnuts.