Valerie said, "some packs are designed so that the hipbelt sits ON TOP of your illiac crest (just below your natural waist). In that position, the belt, when tightened, should be prevented from slipping down by the large bones of the crest."
This is what you need.
ULA and Kalais belts use essentially the same "four buckle system", which helps a lot to "pocket" the hip bone on the belt. However, close examination reveals that the pull on the four buckles is equalized as they are all routed through a single point attachment (dead center on the front).
McHale goes one better, using two actual buckles, allowing for a differential between the tightness of the top and bottom of the belt. With this system, if the top inch of belt width is above the illiac crest, it can be tightened there to prevent slippage, which the lower half of the belt can be a bit looser.