My harness waist strap is at navel level. My weight belt (rubber free diving belt) is just below the waist strap. The crotch strap lies over the weight belt.
The rubber free dive belts are great, are meant to be worn low, and work for most any body types.... just visualize all those skinny male spearo's that wear them.
If all you need is 10 lbs of ballast total, you could conceivably just carry it on your rig (in pouches on the cam bands), but I personally like to have some ditch-able weight for possible surface emergencies.... I have 8 lbs of my total ballast on my weight belt, the rest is "fixed" in my heavy back plate and extra heavy STA.
I do recommend giving the rubber weight belts a try, they are not super expensive (search on Amazon), are comfortable, and CANNOT be accidentally released, yet will release with a single pull if needed.
Best wishes.