A frame with heavy belting accross would conform to the shape of the tank. Many years ago, our boat dolly utilized surplus fire hose for the slings, were still in use when the cottage sold thirty five years later. Wasn't left out in the sun for extended time though.
Did you click on the part number for the 500 gal cradle and look at the blue print ??
Just remember, your tank when full of water will weigh over 3200 lbs, plus the weigh of the cradle.