When I climbed Denali several years back, I towed about 60 pounds in a children’s snow sled. I started with the cords rigged up to my climbing belt, which I had packed for roping up the Kahiltna Glacier, but that first day I was miserable聴the belt pinched and felt like it was made of iron. The next day I rigged the harness to loop around the waist belt on my Dana Design backpack, a much better solution because of the extra padding on the belt. Additionally, my torso played a larger role in tugging the weight.
So, my advice is to wear a good-quality daypack for the essentials (I assume you want the bulk of the load in the sled) and use its padded belt as the harness for the sled. Simply devise a way to loop material (flat nylon webbing would be best) around the belt on both sides. If you snug the belt a little, friction will do the rest and keep it from slipping back. The pack itself also provides a “stop” for the loops.
Campmor, by the way, sells an after-market padded hip belt that could be worn alone and used as the attachment point. It’s only $20. But I think a belt alone will prove uncomfortable, as I learned on my Denali expedition.
Incidentally, I don’t actually recommend hauling a sled. It’s true that pulling a load is easier than carrying it. But rope or webbing traces will let the sled smack into the back of your legs when going downhill. While generally better, metal traces might be too hard an attachment in some conditions. Anyway, I wish you well in your sled-hauling endeavors.