There are a few tricks that few mention about wearing weight belts to prevent rotation. First, several have mentioned the rubber belt, and I’ve been using one for maybe 30 years now. The tightening buckle also would work, but there are other things to consider.
First, the belt needs to be symmetrically loaded with weights. That means that each side needs to have the same weights in the same location.
Second, those weights should not slip on the belt itself. With a web belt (nylon 2 inch webbing), you can thread the weight half way through, make a 180 degree turn in the webbing, so that it crosses itself, then thread it back through, ensuring that it is precisely where you want it (this probably doesn’t work with pouches). You can also secure them in place in a lead weight with a screw, or you can place a stainless two-slot keeper over the weight. The idea is that once it is on the belt, it stays there. You’ll need to make a space for the tank too.
Now one other thing; the person in charge did not give you good advise about over-weighting yourself. Wear what you normally do to maintain neutral buoyancy throughout the dive, and use that amount of weight.
One other thing; the weights need to be jettison-able. You need to be able to dump them. A harness may not give you that option, depending upon how it is worn and what is over it.
SeaRat