Google split shaft collar. An engineering piece for positioning things on rotating shafts on various pieces of machinery. Available in steel and stainless, the bigger ones aren't cheap but probably about the same price as the smaller weights for stretching. The biggest weight seems to be 40mm ID, which isn't big enough for me either so i've had to go down the engineering route.
3 45mm split shaft collars with a cushion ring below. Takes the edge off and makes the stack longer.There's also a shackle on the top of that stack just for a bit more weight.