A 55 gallon drum filled with concrete.
I don't think the weight in the bucket is what's important, as someone mentioned about, the important thing is the base needs to be wide, and not flat. By not flat, what I mean is that it's best to contact the ground out at the edges not in the middle. The tripod idea is best. You could add some weight to the tripod, and if the legs are wide enough, probably a small bucket of concrete hung from the center of the tripod. Would possibly be enough.
Basically, if you've ever tried to make furniture to rest on a rug, if you make the bottom of the furniture flat, it will wobble, even if the rug floor is perfectly flat. Usually furnature that sits on the four corners of the base is stable.
I have a movable 3' Primestar dish that I have mounted on a log that is about 15" in diameter by a 15" high (approximately. The bottom of the log is cut flat, but no matter how flat the ground, it wobbles. What I usually do, however, is drive 3 wedges under the log so that it is picked up slightly off the ground and contacts the ground only at 3 places. It's actually relatively stable. It's my AMC21 dish, and hasn't moved since AMC21 went up. I had previously used it for AMC3, but had to drag it about 15' to get a partial view of AMC21.
My log wouldn't hold a bigger dish, but I think if the base was wide enough, you could do it with not a whole lot of weight. Another option.... I once mounted a dish on a utility trailer that I have. If it has a wide enough wheelbase, it should be stable enough, and you could load up the trailer with rocks for weight, and use the hitch part to level it.