The fpvmanuals Prop Stop is a precision, laser cut piece made from black Delrin. The item is flexible enough to not prevent the props from folding, yet is strong enough to provide resistance against the props folding too far in (or out). Your prop middle piece (yoke) should not be longer than 40mm. I suggest the Graupner folding spinner that is either 34mm or 38mm, but many other folding props work as well. The kit contains 2 prop adapters, which you can use for the front and back, or stick them together if you require a stiffer resistance. You will need to size these pieces to fit your middle piece by cutting them to size with good sizers.